This is a shallower spot closer inland which actually consists of two islands Koh Rok Nai (inside) and Koh Rok Nork (outside). The uninhabited islands have some of the most stunning beaches in Thailand and are popular with day tripping snorkellers. Diving here is also very shallow but this results in good visibility, and an abundance in colourful marine life. According to reports by local media the Koh Rok reef system was not affected by the tsunami.
Another shallow spot even further inland is Koh Muk, you can dive here though the island is more suited to snorkelling. It can get busy on Thai holidays where you'll see an almost comical array of fully clothed snorkellers.