The Similan Islands which are reached from Khao Lak are quite simply the best diving in Thailand and also frequently ranked as being amongst the worlds finest. A cluster of nine islands rich in coral reefs, drop offs, archways and an impressive array of marine life. Tropical fish are here in large numbers but you may also spot a leopard or whale shark. Trevally and jacks are also sited alongside many rays on fantasy reef at Island number eight.
Here's a great Phuket Blog that also covers other places in Thailand.