Koh Rung Sanloem
Koh Rong Sanloem is changing fast. Photos and descriptions from two years ago will be out of date. There are two main places to stay: Saracen's bay or Mpei bay. There are also private one hostel only beaches on lazy beach or Mad Monkey, these may require direct transfers or a lot of walking.
Saracen's bay is no longer the unspoilt paradise it is often described but it's definitely not the circus of Koh Phi Phi for example. The is no stretch of undeveloped beach along the long bay. The hostel are spaced apart and very few encroach on the beach. More of a resort with no Khmer families seeming to live there. There are hotels with infinity pools and $15+ mains. Still a few dorms and rooms below $20 but few cheap eats, it's backpacking days may be numbered.
Mbei Bay is much more rough and ready, a decent amount of construction and the main beach could do with a litter pick. Hotels all backpacker focused and cheap eats a plenty. Nice village feel and Khmer families living there.
Koh Rong Sanloem can be reached by boat from Sihanoukville in around an hour, don't always leave on time but generally punctual for island time. $10 each way, most will only want to sell you a $20 open return. Currently many tour operators offer daily trips to the island. There are also multiple fast ferry options available. EcoSea Dive  offers overnight trips to their bungalows on the northern part of the island and The Dive Shop  runs a daily ferry from Sihanoukville to the sunset beach located on the western side of the island.
You have two choices in your style of boat to the island. You can make it into a day trip and go on either of the cruise boats Party Boat or Sun Tours. These take a whole day to get there but its an enjoyable ride, including lunch, snorkeling trip and drinks at the bar onboard.
That is fun but if you don't care about that and just want to get to the island as quickly as possible then its best to go on the fast catamaran which takes only 45 minutes Speed Ferry Cambodia
There is an old lighthouse, some waterfalls and mangroves and some pretty good dive sites around the island
Koh Rong Sanloem is the cradle of Cambodian diving. It's where all the dive companies based in Sihanoukville mostly head when they take divers out. It also serves as mid-way point for dive trips heading all the way out to condor reef.
Mbei Bay hike to Clear Water Bay
Clear Water Bay is a really beautiful beach with gorgeous water and almost no building or person in sight. It takes about 75mins from Mpei Bay. If you look out to the sea from MB head to your right towards the other pier. Walk 20 minutes to that pier when you get to Koh Rong Samloem Villas. The old lady will moan that you are on her private beach so wade into the water to the yellower water where the sand banks is. Wade through the water for about 15 minutes (thigh height) and aim for the headland or the rocks just before it. When you get to the rocks you should look behind them and there is a sign to clear water bay / driftwood. Follow it through the jungle (easy path to follow) for about 30/40 minutes. There is a turning to a tiny beach after 25mins but continue on the main path to clear water bay.
M'pai Bay, although small and quiet, has a high standard of food with local specialities and flavours from around the world.
Saracen Bay offers many restaurants with various kind of cuisine .
Chill Inn, M'Pai Bay, ☎ +855 (0) 86 86 05 22, . You can find this Breezy Beach Bar & Guesthouse in the up and coming Traditional Fishing Village of M'Pai Bay on north coast of Koh Rong Samloem, Dorm Rooms Available.
The Coffee Shop, A nice little place to grab a coffee with amazing views over the bay, with a large selection of coffee`s from all over Asia, also available are freshly baked cookies, cake and pastries.
The majority of places to stay are at Saracen Bay.