Koh Kham is a tiny islet a kilometre off Koh Mak's western tip, fringed with white sand and clear shallow water. The whole island can be walked around in twenty minutes. It is privately held and there is a small entry fee for day visitors, but the snorkelling — bright fish over a healthy coral garden close to shore — is some of the best in the area.
Longtail boats run across from Ao Suan Yai in fifteen minutes, or you can paddle a kayak when the water is calm. Go early; by midday the day-trip boats from Koh Chang arrive and the beach gets busier.