• EcoSea

• Cambodian Diving Group

• Dive Shop Cambodia

• Coral Gardens

• Koh Rong Dive Center

• Scuba Nation has regular dives to Koh Rong Samloem from the mainland.

A half dozen scuba diving companies operate right on Koh Rong Samloem Island, with a few more from the mainland 2 hours away by dive boat.  Waters are shallow, and visibility is usually quite good.

Not so much of the bigger marine life, but lots of diversity.  You also get to stay on one of Asia's most beautiful tropical islands if you want, and can dive right from the shore.