Diving the Cape Peninsula and False Bay/Partridge Point
This article is a travel topic
The dive sites at Partridge Point, including Big Rock, Seal Rock, Deep Partridge and Peter's Pinnacles are inshore rocky reefs at the southern end of the Castle Rocks restricted zone within the Table Mountain National Park Marine Protected Area on the False Bay coast of the Cape Peninsula, near Cape Town in the Western Cape province of South Africa.
Fish, seals and a wide range of invertebrates can be seen in surroundings which can be spectacular in good conditions. This site is at the southern edge of the Castle Rocks restricted zone, within the Table Mountain National Park Marine Protected Area, where no fishing has been permitted for many years, and the result can be seen in the numbers of fish. This site is second only to Castle Rocks for the variety and numbers of fish to be seen at a Cape Peninsula dive site.
The site is large and there are a few areas that are fairly well known. Most of the rest is virtually unvisited by divers.
The Big Rock area has very varied reef profile with a few well known features, and is limited in depth to just over 20m.
Seal Rock is popular with visitors who want to dive with seals. Local divers don't usually bother much with looking for seals as they are generally unavoidable around this general area, and instead head for the deeper parts of the reef east of Seal Rock, where there is some spectacular topography, as well as large gorgonian sea fans.
Peter's Pinnacles are a small section of the site, which is topographically interesting and biologically diverse, while remaining in the depth range for entry level divers.
The last point on the Cape Peninsula's False Bay shoreline north of Smitswinkel Bay. About a 4km boat ride from Miller's Point slipway.
The site known as Partridge Point includes the Big Rock area at the extreme south of the restricted zone, while Seal Rock is the large fairly low rock furthest to the east of the point. Peter's Pinnacles are south west of the big rock, are just outside the restricted zone, and are not identified by any visible landmark.
Most of this site is in a Restricted Area (no take zone), and all of it is in a Marine Protected Area (2010). A permit is required to dive, and spearfishing is prohibited.
The area is marked on maps and charts as "Partridge Point", and the "Seal Rock" is named for the seals often seen basking on it. "Peter's Pinnacles" is named after its mapper. "Big Rock" is the highest rock above sea level of the area. The name "Deep Partridge" is simply descriptive, as it is the deep part of the Partridge Point reef complex.
Maximum depth is about 26m on the sand to the east of Seal Rock, but is nearer 17m at Big Rock, and about 22m at the swimthrough south east of Big Rock on the edge of the reef. Peter's Pinnacles is mostly about 13m on the sand. Parts of the reef extend to the surface, and there are a number of pinnacles of about 6m depth on top.
Visibility can vary like any other dive site in the area, but in diveable conditions is likely to be between 5 and 10m, and on a very good day, up to 20m. It is generally similar to, but often not quite as good as visibility on the Smitswinkel Bay wrecks
This extensive reef is made up of large numbers of granite boulders and outcrops, some very large, and some of these extending above the surface by several metres. The reef is varied, some places it is low and made up of small boulders, rubble and low corestone outcrops, and in other places it is a wild chaotic jumble of huge boulders, perched on even bigger ridges and flatter outcrops, forming overhangs and swimthroughs in a few places. The reef is bounded by sand bottom to the south and east, and to the north by the continuation of the reef complex which extends almost as far as Castle Rocks in the shallows, and is of uncertain extent in deeper water. The reef at Partridge Point is partly divided by a number of sand tongues, and sand patches, which separate the reef into a number of sub-sites, which are described in more detail below.
Partridge Point: Big Rock
This site is in the vicinity of the highest rock in the area. It is also further south and inshore of the other large visible rocks.
A tongue of sand bottom at a depth of about 16m extends northwards to the west of the big rock, which is very sheer on the south west side, with an uninterrupted drop to the bottom near the south end, and to the east of this rock is a deep but narrow gap between it and two smaller surface breaking pinnacles, where large shoals of fish are sometimes seen.
There is a small air trap under an overhang at the north east side of the low rock to the east of the Big Rock. The overhang is several metres long and the air trap roof is at two levels, the lower being at about 6m depth and the upper at about 4m. The outer edge of the overhang droops downwards a few metres in places. This type of formation is characteristic of heavily weathered granites, and probably developed before False Bay was immersed at the end of the last Ice Age.
To the north of the Big Rock and the air trap rock are two small sandy patches, which form the northern border of this part of the reef.
About 50m to the south-east of this exposed group of rocks is a big swim-through opening onto the sand to the south east, at a depth of 22m, with relatively deep reef between the north side of the swimthrough and the air trap rock. This swimthrough is about 3m wide and the roof is about the same height above the sand. The west side opens into a small sand bottomed depression in the reef, and on the north side the swimthrough forks and has another, smaller entry, which opens into a small chimney.
To the east of the air trap overhang rock, and north and east of the swimthrough, are deep cracks and high boulders, with some small swimthroughs and overhangs. This part of the reef bulges out about 50m to the east and is bounded to the north by a small sand tongue. East of that is sand. There may be some more reef further east across the sand.
To the west of the Big Rock a tongue of sand extends in from the south, and to the west of that is more reef, largely unexplored, and which includes sone high pinnacles along the edge of the sand (see Peter's Pinnacles, below). Even further west is another sand tongue which extends north towards the high exposed rock which marks the west end of the great northern sand tongue. There is some low reef to the north of the western sand tongue and then another sand patch which extends almost to the small high rock at the inshore end of the northern sand tongue. The western sand tongue extends to the shoreline reefs on the west side of the site.
Beyond the the sandy patches to the north of the Big Rock is more moderate profile reef, not yet mapped in detail. To the north east is lower reef, extending towards a group of rocks which breaks the surface to the west of the seal rock, and interrupted by a narrow sand patch and short sand tongue.
Seal Rock and Deep Partridge
This part of the reef complex is east of the large but not very high exposed rock to the east of the point, which is often occupied by the Cape fur seals which give it its name.
The eastern edge of the reef is at about 25m on fine sand. The south eastern part of the deeper reef is not very high profile, and has sand bottom between most of the outcrops, but as you get closer to the seal rock, and shallower, or further north, the vertical size of the larger rocks increases. This south eastern end of the reef is also known as the Gorgonian Forest, as there are large numbers of palmate, sinuous and flagellar sea fans, and the occasional multicolour sea fan. Between these are the usual dense aggregates of sea cucumbers, hydroids, strawberry and striped anemones, and feather stars.
There is a group of rocks with an arch swimthrough at the end of a gap between outcrops, and a large low headroom cave at S34°15.331' E018°29.016'. Further inshore the bedrock slopes up gradually for a while with ridges and gullies, and at S34°15.327' E018°28.980' there is a pinnacle extending to within about 5m of the surface, which is a convenient place to ascend if the surge is not too strong. There are several other, mostly lower, pinnacles scattered around this part of the reef, and many have not been mapped yet.
The reef edge further north swings round to the west in a large arc where the very long northern sand tongue interrupts the reef and forms the northern border of the Partridge Point Seal Rock area. The sand tongue does not appear to be very wide in the deep parts, and more reef can be seen to the north in good visibility, but this area has not been mapped (June 2010).
A large part of the north eastern part of the reef is higher than 15m on top, and there are places on the lower part of the wall that are covered by a dense mat of colourful Hairy Brittlestars, which give this area the alternative name of the Flokati Rug.
The northern sand tongue is much wider to the west, and then tapers down to a narrow point which swings south-west and ends right at the small but quite high rock to the north of the Big Rock.
The reef just to the east and south of Seal rock is quite shallow, and here you can be almost certain of being visited by seals, which is a big drawcard for tourists and visitors to Cape Town, but not particularly interesting to local divers who see seals often. The seals will buzz a diver and bark at you, but have not been known to bite without provocation. They can be entertaining for a while until you start getting dizzy from trying to follow their movements. The reef cover in these parts is the typical combination of Red Bait ascidians, brown seaweeds, kelp and coralline algae, with anemones, sea urchins etc where they can find a gap.
Position S34°15.515', E018°28.687' (top of the main pinnacle)
To the south west of the Big Rock, and across a tongue of sand extending from the south, is an area of reef relatively unexplored before 2010, but now fairly well mapped. This is a moderately shallow area (max depth around 17m at the south eastern edge, but mostly about 13m on the sand) of boulders and outcrops, jumbled together to form some quite spectacular structures, with pinnacles, steep-sided gullies, overhangs, small caverns and swim-throughs.
To the west of the pinnacles is another sand tongue extending to the north and separating the pinnacles from the shoreline reef.
The main pinnacle to the south west of the site is a granite tor extending to within about 4m of the surface at low tide, and about 30m wide from east to west at the base. It has sparse kelp on top and this is visible at the surface at low tide. An idea of the structure can be gained from the illustration.
There is a small cavern at the east of the base of the main pinnacle formed by a large boulder resting on top of other boulders, with a gap below. The cavern is about 5m wide both ways, and about 1.5m high. There are two main openings: The western access is via a narrow slot between the base of the pinnacle and a lower boulder about a metre away to the south. This slot curves to the left as it goes under the wide flat boulder which forms the roof of the cavern, and opens to the cavern proper at this point. Directly along the continued west wall of the cavern is the rear or northern opening, which is quite small, and a bit tortuous, and may not be big enough for some divers. The south opening is almost the full height and about a metre less than the width of the cavern, and is almost at the reef edge. The floor is also rock, fairly flat, at 13.2m, it is about half a metre above the sand. Many lampshells can be found inside the cave, as well as a couple of gorgonian fans.
The main outcrop of the pinnacle is nearly sheer sided with some overhang. on the west, south and east sides, and has a deep indentation on the north face. To the south is sand. To the west is a group of large lower outcrops with a couple of small tight swimthroughs. To the east is a steep high outcrop and a few lower rocks, then a large patch of sand.
About 30m to the north, across an area of mostly low boulders with sand between them, are the northern pinnacles, a slightly larger but perhaps less impressive stack of boulders, also extending to within 6m of the surface at three places, and also with a small swimthrough with a hidden opening on on the south face, and a more obvious opening to the north at about 10m depth. These pinnacles also have sparse kelp on top, which may be visible at the surface at low tide.
To the east of the northern pinnacles are three high outcrops which extend to within 9m of the surface. The reef to the north and east of these pinnacles is lower and more rounded.
In the far east of this part of the reef there is a tunnel under a huge boulder which may be big enough to swim through. This boulder is 7.5m deep on top. The tunnel has a sand bottom at about 16.3m and is probably about 6m long with a triangular section.
To the south east of the main pinnacle are a series of rocks which include the south eastern pinnacle, a massive outcrop which extends to within 6m of the surface at low tide, and a number of lower outcrops, mostly 1 to 3m high above the sand.
Marine life is extensive and varied. The reef is filled with life and colour. When one looks up on a day with good visibility and the reef is silhouetted, the sun filters through the kelp fronds on top of the reef, and the schools of fish in midwater make for stunning underwater scenes.
Other parts of the Partridge Point reef complex
There is a swimthrough with a small air trap under the north and west sides of the exposed rock marking the inshore end of the great northern sand tongue., and a smaller low air trap overhang under the south side of the same rock. This was found years ago by divers doing shore dives from the point, and has been mapped as Dave's Caves.
Areas bordering on Partridge Point
To the south west, the reef extends a short way along the coast towards Smitswinkel Bay beach, where a major geological fault forms the boundary between the northern granite reefs, and the southern sandstone reefs. There is still granite present to the south, but it is below sea level for several kilometers, whereas from Partridge Point northwards, the shoreline and lower slopes of the mountainside are granite as far as Simon's Town.
There is an isolated exposed rock south of Peter's Pinnacle, sometimes known as The Anvil, due to its shape, and not to be confused with Anvil Rock on the SAN charts situated off Cape Point. The reef around this rock is said to be small.
To the north of the point, and of Seal Rock, there is the great northern sand tongue, curving in an arc from the deep sand bottom at about 25m, all the way to an exposed rock between the point and Big Rock. This is narrow in places and wide in others, and on the north side it blends into low broken reef, where it is difficult to draw a line between sand with rocks and rocks with sand. This can be seen on the main site map. To the north of this is more extensive rocky reef, mostly moderate to low profile, but not well described or mapped at this stage, and possibly extending continuously to join the Finlay's Point reefs. Further offshore to the north is the dive site Finlay's Deep.
This area is not often dived by recreational divers, but is part of the Fish and Invertebrate (FIN) survey core area. Considerable shoals of fish may be seen, and will sometimes follow a diver around. Hottentot seabream, Roman, Galjoen, Blacktail, Fransmadam and Zebra may be seen here, and also puffadder shy sharks.
Geology: Granite corestones of the Pre-Cambrian Peninsula pluton, and sand. Much of the sand is fine quartzitic sand typical of the Cape Peninsula beaches, but there are also scattered areas of gravel and shell. The unconformity between the granite and the overlying sandstones is a short distance above the road at this point, and can be clearly seen from the road at the corner just above Smitswinkel Bay, where the road turns inland towards Scarborough. The sandstone dips quite steeply to the west here, and slopes down to the roadside a short distance around the corner. The shoreline reef also has various sized boulders of Table Mountain Sandstone, which have broken loose from the mountainside over the millenia and ended up at the shoreline, where they have been weathered further, and the smaller ones rounded by surf action. When covered by benthic organisms, they look much the same as the granite boulders.
A peculiarity of the weathering process of granite leads to the development of domed top cavities under corestone boulders, possibly to to prolonged intimate contact with damp and probably acid soil. When these boulders are later immersed in the sea due to sea level changes, the decayed rock (saprolite) is washed away leaving the domed roof cavities which can trap exhaled air from divers, or in rough weather, air entrained in the water resulting from breaking waves at the surface. These air trap caves when holding a pocket of air, have a visibly shimmering roof, and sometimes the air space is large enough for divers to stick their heads up into the gas pocket and talk to each other. It is not known exactly what the quality of the air in these pockets is like, so it is strongly recommended not to inhale it at any time, as it may be oxygen depleted, and passing out underwater is not good diving practice.
There are at least three air trap overhangs at Partridge Point.
The site is often at its best after a north west wind, but may be good even during an easterly if it is not too strong and the swell has not built up yet. Surge may be considerable in long period swell even if low.
The site is exposed to south east wind and waves, and is moderately exposed to south westerly swell, particularly long period swell, which bends round Cape Point. A SSW swell with a medium to long period will hit this site hard. The site is usually at its best in winter but there are also occasional opportunities in autumn and spring, and sometimes even in summer a good day may occur.
Look for days when the forecast is for swell from the west, and preferably low. This can often occur just before a cold front gets to the peninsula, and in these cases the weather is often mild, even sunny, and with little wind in the day or two before the front arrives. During the passage of the front, the weather is generally windy, overcast and frequently rainy, but may still provide very good diving if the wind is not too strong for the boats to operate, as it will be offshore until the front has passed.
After the front has passed, in winter one can expect a day or two of good diving before the next front. In summer, a front is commonly followed by strong south easterly winds, which kick up an unpleasant short onshore chop which makes the boat ride uncomfortable and messes up the visibility.
Access is problematic for shore dives as it is necessary to cross through private land for the most direct route. Also it is a long walk down from the road, which is 100m above sea level at this point, followed by quite a long swim to get to the popular sites. Access from Smitswinkel Bay requires the same climb, but a longer walk over a boulder beach, or a very long swim.
Because of this difficult shore access, this site is almost exclusively accessed by boat: Partridge Big Rock is about 4,2km from Miller's Point slipway. There is good anchorage in westerly winds to the south west of Big Rock on a sand bottom in about 17m depth. There is also good anchorage on sand at about 13m to the south and west of Peter's Pinnacles. Dives at the Seal Rock and Deep Partridge areas are usually done as live-boat dives, particularly if there is much surge. Picking up divers in this area can be tricky if they surface close to the rocks, as there can be a surface drift to the north caused by waves breaking over the reef shallows.
Anchoring on the reef is considered disrespectful of the ecology, as it tends to damage the reef animals. Shotlines should be used with care, as there are many places they may snag, and either damage the reef or require a diver to free them.
There are extensive kelp forests in the shallower areas, and a heavy cover of invertebrates on the rocks. Large shoals of small fish such as Hottentot sea bream and Fransmadam are frequently seen, and smaller groups of larger reef fish such as Roman, John Brown, Two-tone fingerfin and Galjoen. Cryptic species such as klipfish and redfingers are fairly common.
Cape fur seals often visit divers at this site. They also spend a lot of time basking on the exposed rock named Seal Rock, or floating around in small groups with their flippers waving in the air.
Large areas of reef are covered in red-chested sea cucumbers, sometimes so densely that the rock is not visible between them.
Big Rock is a good place to go to find nudibranchs: inkspot nudibranchs, Cape dorids, and all three known local species of Tritonia have been seen here. At the swimthrough is a very photogenic whip fan, often attended by fish, and in the crevices inshore of the swimthrough is a lovely profusion of palmate and sinuous sea fans. Heading from the swimthrough east along the reef edge, many octopus can be seen in the cracks of the rocks. Pipefish can also be seen at the reef edges and on the sand and false plum anemones are common on the reef. Look out for walking anemones eating multicoloured sea fans, and for territorial male romans in crevices and overhangs. Hermit crabs are abundant and empty shells are hotly contested.
Seal Rock and Deep Partridge
Seals are an obvious feature of the shallower areas of this section of the site. Interesting nudibranchs, such as an undescribed species of Eubranchus, have been seen here, along with sea spiders, all hidden among densely packed multicolour seafans, Cape urchins, and colourful sponges. Heading for the Deep Partridge section, large numbers of small nippled sea fans, and larger sinuous, flagellar and palmate sea fans can be seen. The deeper parts of the reef tend to have larger gorgonians, particularly the deep reefs to the east of seal rock and south of the great northern sand tongue.
In the deep section, the large gorgonians are often attended by basket stars, Cape long-legged spider crabs and hotlips spider crabs. They are the favoured haunt of blue-striped hermit crabs, and the favoured food of ornate topshell snails. Bluespotted klipfish are often found resting on the palmate sea fans.
Some sections of the deep reef are densely covered with brittle stars. Others are heavily encrusted with strawberry anemones, redchested sea cucumbers or striped anemones. Worth looking out for among the striped anemones is the toothed decorator crab, which uses the anemones' habit of shooting sticky defensive threads through their body walls for its own defence when it is vulnerable post moulting. Elegant feather stars are the home to two newly described commensal species: a tiny myzostomid worm and a smallish shrimp, both worth looking out for. Unusual fish such as Cape and bluefin gurnards are sometimes seen. A not specially unusual but very well camouflaged and hence rarely seen fish is the smoothskin scorpionfish. They are spottable by the sharp eyed diver, usually because of an incautious movement on the fish's part.
Being a much shallower section, there are few large gorgonians in this area, but many multicoloured sea fans. Reef tops may be covered by a dense layer of the sea squirt known as red bait, and in these areas there is also often a dense turf of upright coralline algae.
Otherwise, serpent-skinned brittle stars are common, as are spiny starfish, vividly coloured sponges, feather stars in dense mats and bushy moss animals. Sinuous sea fans are present in the deeper areas, often serving as an attachment for shy shark egg cases (mermaid's purses).
The kelp beds and rocks in the area north of Peter's Pinnacles, at the inshore end of the great northern sand tongue ere fairly shallow, not usually exceeding 8m depth, and would be a good area for snorkelling. This area also includes a number of small air trap overhangs , holes and swimthroughs, Particularly the two air traps under the low exposed rock just at the extreme tip of the great northern sand tongue, known as Dave's Caves, after the diver who reported their existence to the survey and mapping team.
Big Rock area:
Seal Rock and Deep Partridge:
Good site for photography. In good visibility a wide angle or fisheye lens will give some spectacular views of the reef structure, and there are always lots of small invertebrates to keep the macro photographer entertained. There is usually enough suspended matter in the water to make backscatter a problem, so for anything other than macro work, an external flash is recommended.
Big Rock — Start at the west side of the Big Rock, and descend to the bottom. Swim south east along the edge of the reef, round the corner at the big boulder perched on the reef, and then east as far as the swimthrough. Spend a little time exploring the swimthrough and environs, then head north-west until you get back to the area around the Big Rock. swim with the reef to your left, and with a bit of luck you will come to the small sand patch to the north of the exposed rocks, and the air trap overhang fairly high up the north face of the lower exposed rock, at a depth of about 6m. It can easily be recognised by the narrow curved drooping part of the overhanging rock, a little like an elephant's trunk from some angles. If you have time and air to spare, swim west and cross the gully between the air trap rock and the Big Rock. Continue around the Big Rock along the western wall, and if you have the time, look into the gully between the southern end of the Big Rock and the south eastern exposed rock. Ascend slowly and try to time your ascent to be near the boat if it is anchored.
Deep Partridge — Dives usually start on a shotline set towards the deep end of the reef, and spend a bit of time along the edge of the sand, after which it is suggeated that you work your way west towards the shallower areas until the surge starts to become a problem, then move away over the deeper water to surface.
Gorgonian forest — This can be approaced either directly by dropping a shotline in the area, or by following the reef edge to the south-east corner and then moving north west onto the reef about 20 to 50 metres. This is a large area and not easy to miss.
Flokati rug — This area is the lower part of the reef edge to the north of the deep sector, and was named for the dense cover of brittle stars. You can find it by following the reef edge from the north sand tongue to the east, or the east reef edge toward the north. After visiting the brittle stars, work your way up to 15m and follow the contour to the north west for a pleasantly rugged topography with a few large overhangs and moderate pinnacles.
Peter's Pinnacles — This site is most conveniently dived by locating the main pinnacle with a fishfinder or by GPS, but at low tide the kelp on top may be visible. Drop in at the top of the pinnacle, which is at about 4m depth at low tide, descend on the south side and visit the cave, which is quite easy to find, and just above the sand. To the west of this pinnacle are a pair of small swimthroughs on the sand bottom, and about 25m north is another group of pinnacles, extending to within 6m of the surface. There is a tight swimthrough from the south to the north under the largest of the pinnacle boulders. The north entry is easier to find, as it is directly below the large boulder. The reef to the east of these pinnacles is high profile, with three rocks extending above 9m depth in a sparsely scattered row bearing roughly south east and following the line of the reef edge. About 30m east of the last of these is a huge boulder perched on the reef just above the sand with a narrow but fairly straight triangular section tunnel under it.
If you still have enough air you can then swim across the sand patch about 70m on a compass course of 210° magnetic from this pinnacle to the south eastern pinnacles, and then follow the row of lower rocks north west about 60m to get back to the main pinnacle. The round trip is about 300m point to point.
Prow Rock — Start at the southern edge of the reef, preferably directly south of the small blinder to the east of Seal Rock, but otherwise about south east of Seal Rock. Follow the reef edge to the east while keeping a lookout to the north. Stay a few metres over the reef as the Prow Rock pinnacle is about 10m north of the sand edge. The rock is quite high and vertical on both sides and looks something like the bow of a shipwreck. After this you can move northeast over the reef to the Gorgonian Forest area, and with luck may find the Squash Court, a rock formation with two high, almost vertical walls on two sides of a flattish outcrop, with the third side somewhat lower.
Seal Rock — Divers who want to see the seals are usually dropped off to the southeast of Seal Rock, preferably near but not too near to a group of seals gathered at the surface. Seals tend to hang out over a shallow spot on days when the swell is low, as this probably gives some protection against predation from underneath, so the boat should generally not try to get too close. Get into the water, move a few metres towards the seals and descend to just above the reef, and wait for the seals to buzz you. It is difficult to take still photos as they move very fast, and change direction often, but wide angle or fisheye video often gives good results.
Strong surge in shallow areas and breaking waves over exposed rocks. The severity depends on the sea state, and it is usually not difficult to avoid the rougher areas.
The surge at this reef can be strong enough to wash seals off the seal rock, although they are more than 2m above the water, and the rock is about 50m wide. This would be considered marginal diving conditions, and on such days divers should stay a safe distance from the rocks, and stick to the deeper parts of the reef.
The site has several fairly large rocks and groups of rocks extending above the water. It is possible to surface on the far side of a rock from the boat, and it may take some time for them to find you. This is normally only a problem if you are getting hypothermic, are unfit, or can not provide sufficient buoyancy due to overweighting or inappropriate equipment. Try to remain aware of where you are in relation to the layout of the reef, and let the skipper know where you are likely to go and how long you plan to dive, so the crew can keep a lookout in the most likely places. A DSMB can be inflated on the surface and held up as a marker to indicate where you are. Be prepared to spend a little time on the surface waiting for the boat, as they may have to pick up other divers before you, and they may be behind another rock.
Certification suitable for the planned depth is expected. No special skills are required. Reasonable buoyancy control is desirable to avoid damaging the reef life.
A light will be useful for looking into crevices and overhangs. A compass will help to keep track of where you are, and is useful for navigating the routes described above.
A DSMB may be useful at the Seal Rock area if there is a lot of surge, as in these conditions it is not recommended to try to do safety stops over the pinnacles, and the shot line may have to be in deep water. A DSMB is also useful to attract the attention of the boat when you surface, as the site is large and the boat may be on the other side of a rock.
Nitrox can significantly extend no-decompression time at the deeper areas.