You can pretty much just walk in the water from any of the beaches to see some tropical fish. The best snorkelling is off Sunset pier and around the north side of M'pay Bei. Here you can see a wealth of tropical fish and coral. Most places on the island will hire you a snorkel for around $3 a day. Some places have snorkelling trips around the nearby area. If your guesthouse doesn't offer this, they will point you in the right direction. With many dive sites in the area only a short boat ride from Koh Rong Saleom, it makes an excellent base for diving. Most dive shops pick up people/gear from here meaning you avoid the 2-hour boat ride each way on the dive boats. So be sure to check out the diving page also.