Hydra is deservedly one of the most popular day-trip destinations from Athens. The port of Hydra has a scenic location in a deep harbor, with whitewashed houses rising on the hills on both sides from an azure blue sea. An enlightened policy of development has meant that there are absolutely zero high-rise houses, cars, or motor-bikes -- the only internal combustion vehicles on the island are the few city-owned trucks and a fire truck -- and while the main port boulevard is packed full of tourists in season, only a few minutes away are quiet back alleys with nary a soul, and many of those visitors are day trippers, leaving the town relatively peaceful by night.
Incidentally, the name has nothing to do with the nine-headed monster of Greek legend; instead, it's an abbreviation of Hydrea, or "spring". The spring has long since run dry, but the name remains.
All hydrofoils and catamarans dock at the center of the north side of the island at the eponymous port of Hydra. Practically everything of interest to the visitor is on the waterfront, in the few main streets behind the harbor, or along the main road that follows the coast or within a block or two of it. If you head west for about a kilometer, past a little headland, you'll reach the neighboring, far less touristy village of Kamini.
There are frequent boats between Piraeus and Hydra. You can opt for either a slow car ferry (around 3 hours) or a fast catamaran (1.5 hours). As of 2007, the cheapest fast option is Euroseas , which has one daily departure at 9:30 AM (return at 6 PM) and sells return tickets for €34. Hellenic Seaways has more frequent sailings (up to half a dozen daily), but also charges more at around €42. Due to the popularity of Hydra, it's possible for some of the Hellenic Seaways boats, which are small, to sell out during peak periods, especially the ones at the most convenient hours. Tickets for all boats may be bought from agencies at the harbor.
Motorized transportation is forbidden on Hydra. Getting around the town center on foot is easy enough, but for going elsewhere the only options are by donkey or boat taxi, both of which congregate at the center of the port. For the boat taxis, there's a sign (in Greek only) listing fixed fares for popular destinations, ranging from €9 for the short hop to Kamini to €120 for a circuit around the whole island.
Hydra doesn't have much in the way of sandy beaches, but there are plenty of smooth rocks and pebbly bays for sunbathers. The water is clear and local daredevils show off by diving off the cliff on the road to Kamini — don't try this unless you know what you're doing!
The only good swimming spot within easy access of town is the rocky shore called Spilia in the Kamini district on the west edge of town; just walk along the main coastal road leading west for about twenty minutes, and it's just past the large cafe-bar. This area isn't really a "beach" but a rocky area which has been slightly developed by the addition of a few stairs and level places in the rocks which are good for sunbathing. To swim, you need to climb in and out of the water via some rather primitive ladders set in the sea wall (and beware of sea urchins on wall,) which might be difficult for some people. The water, however, is clean, and usually calm except when a boat sails by close to shore. The view is beautiful.
Hydra has a bit of a reputation as an artist's haven, and there are plenty of galleries around.
Hydra offers many restaurants. Most prominently feature seafood, but little asterisks in the menu note that pretty much all of it is actually imported frozen from far away. Only a few restaurants offer local fish, and they charge accordingly.
For a more local experience, head down to Kamini, which has a few distinctly unhip tavernas populated mostly by grizzled Hydriots.
There are plenty of little hotels and guesthouses in the port of Hydra and even up and down the alleys of Hydra for those who don't want to limit their visit to a day trip -- as well as many private home rentals offered both locally and by private villa/vacation rental agencies to be booked in advance. Note that some Hydra accommodations may be accessible only by climbing stairs, so if this is a problem for you, ask when booking.
The island has several free Internet cafes in the harbor.
The flip side to Hydra's environmentally friendly transportation policy is the presence of large amounts of what might be termed "donkey exhaust", so watch your step when walking around.
Many ferries stopping in Hydra continue onto Spetses, just 30 min away.