The most popular island trips are a half and full day lunch and snorkeling trips to one of the closer islands such as Koh Tas, Koh Rung Samlem and Koh Russei. Overnighting on the islands is possible with Koh Ru and Sankeor offering rustic bungalows and beach restaurant bars on Koh Russei and Koh Rung.
Local waters are relatively warm but visibility is often low, perhaps due to fresh water runoff. Visibility is better around the more distant islands and Condor Reef. Still, the snorkeling can be pretty good around the nearby islands with rocky reefs offering shallow water corals and colorful fish. The best scuba diving is around the more distant islands, but the area has not been thoroughly explored, and the best diving spots are yet to be identified.
The closest island is Koh Pos (Snake Island), 800m off Victory beach. The beaches on Koh Pos are pretty average. The nearest island with reasonably good beaches and snorkeling is Koh Kaong Kang (Koh Tas), located about 45 minutes to 1 hour away.
Koh Rung Samloem is the nicest island within daytrip distance, taking 2-2½ hours one way. It has 8 or 9 beaches including some very nice, sheltered beaches on the north side near Koh Koun. Rocky reefs around both islands harbor a variety of sea life in both shallow and deep water. Good for divers and snorkelers alike.
When the wind is from the north (Nov-Feb), it is often better to go to the islands to the south. They are sheltered by the mainland, which makes for calmer but siltier waters. For snorkeling, Koh Khteah, Koh Chaluh and Koh Ta Kiev are best. Koh Chaluh is just a rock but it has some nice coral and there are often a lot of fish. There is good snorkeling on the north side of Koh Ta Kiev where giant mussels live. Koh Russei (Bamboo Island) has some decent beaches and bungalows run by Bar Ru.
Koh Thmei and Koh Seh both lie within Ream National Park. Some tour operators as well as the National Park offer boat trips to both islands, though the beach on the southwest side of Koh Thmei is the most often visited.
The most promising scuba diving waters lie 4-8 hours to the southwest, around Koh Tang, Koh Prins and Poulo Wai. The distance requires at least a nights stay. The area has rocky reefs, depths of 15-40m and better visibility than reefs closer to shore. Koh Prins has two shipwrecks to the northwest at 30-40m. Koh Tang played a major role in the Mayaguez Incident and still shows the scarring of the May 1975 battle between American and Khmer Rouge forces.