Koh Samui and neighbouring Koh Pha Ngan offer some great tropical dive experiences. Corals here are good as is the range of marine life which includes whale sharks at the Sail Rock site. All levels of divers are catered for.
Koh Tao is considered by many to be 'over-dived' and therefore a little spoilt now i.e. damaged reefs. It is however a fairly remote idyllic tropical isle offering many water sports including diving and snorkelling. The snorkelling here is very good and can be better than diving at times.
Here's a great Phuket Blog that also covers other places in Thailand.