Nearby islands have some fairly good snorkeling opportunities, and the water is warm. You can see some colorful fish around some of the coral near the rocky reefs. The close island to Sihanoukville is Koh Pos (Snake Island), which is only 800m from Victory Beach. Koh Pos is a little rocky but is convenient. Koh Koang Kang (Koh Tas) is a little further away (1 hour) with good beaches and snorkeling.
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In the same direction but further is Koh Prins. Around here there are reefs, depths of 15-40 meters and the water is clear. In addition, there are two shipwrecks. Poulo Wai is an unexplored but really exciting destination – locals claim they see manta rays. It is 145km southwest of the town, around 8 hours, and is possible if you talk to tour operators. You may have to have long discussions first
Condor Reef is 75km west, and means another long trip (usually three days minimum), with camping on Koh Rong which is the nearest land (the Reef is 35km west of here). Diving is great, with high visibility, and incidence of reef sharks, turtles, moray eels and a lot more. You can also find pieces of ancient Chinese porcelain and shipwrecks. Koh Rong has wonderful empty beaches, including one which stretches for 5km with nothing to be seen but sand and sea.
There are many different places offering trips to the islands. It is best to organize this through your guesthouse or through an agency in town. If you are a serious diver, contact one of the diving outfits in town; some of the further afield places will need some negotiation. In addition, the further places can end up expensive (few people go and the waters are uncharted, so you are paying for the privilege of being one of the first ever to go to these exciting places!) Some outfits set a minimum number of people (5-10, for example) for diving trips; even if you are going to the close by islands, and just want a picnic, you will find that it is much more expensive alone, so join up with some other tourists if possible.
Trips could include transportation, boat, equipment, food and camping (if necessary), so make sure you ask about all of these areas. Prices vary hugely.
To Koh Pos, which is nearby, the price is around US$10 round trip, depending on the amount of people. Koh Koang Kan (Koh Tas) is a little more expensive. For the islands to the south, Koh Kteah, Koh Crhalo and Koh Ta Kiev range in price and can be combined. Bank on around US$10 for a day trip, and the same price for Koh Russei. Koh Rong Samlem is around US$70 per person for divers and around US$30-40 per person for non-divers.
Koh Tang gets more expensive at around US$110 for divers and around US$70 for non-divers (this is a three-day trip). Going on to Koh Prins and Poulo Wai, expect to have to hold some discussions and for the price to go up substantially. Koh Rong is around US$130 for divers and US$80 for non-divers.