Koh Seh Island is a great place to relax, or to explore.  You can go snorkeling, fishing, swimming and boating around the island.  There is no public pier on the island, so you'll have to swim or have a small boat take you to the beach.

Scuba Diving is one of the most popular activities here.  Whether you are just learning, or an experience diver, many dive shops located in SihanoukVille can help.  You need to be aware that you are only minutes to the island of Phu Quoc, Vietnam (that's another country) and don't want to get caught in these waters without proper documentation and permissions.

If you go out to Koh Seh Island, watch out.  Kilometers of as yet unspoiled tropical jungle.  No lions, tigers, or bears, but there are the usual island bugs and birds and other things to beware.

You can check out the other islands around SihanoukVille.