Koh Phuket is the largest dive centre in Thailand and the main base for liveaboards heading for destinations in the Andaman Sea. There are a few good dive sites around the immediate coastline of Phuket Island itself, since the marine life and visibility have been heavily affected by offshore tin-dredging operations.
Conditions improve considerably around the outcrops, islets and islands off the south of Phuket; the currents here carry away algae and silt and the corals have more chance of flourishing. The reefs are generally in a healthy condition with good hard corals as well as colorful soft corals.
Marine life is plentiful, with visits from large pelagics adding spice to the diving. Some sites are quite deep with ripping currents often present, but there are plenty of other locations suitable for all levels of both recreational and technical divers.
Scuba diving here is big business. The larger centres cater for divers from all over the world; course and dive trips are generally supervised by multi-lingual staff, but English remains the main language. Several larger centres provide a transfer service to and from Phuket International Airport.
Source Reference : Pocket guide dive thailand