I have been diving of Koh Phangan in the Angthong National Marine Park which was pretty good. Saw turtles, baracuda, sea snakes, etc. I have heard from friends though that a liveaboard out to the Similan Islands is the way to go. You should be able to get one pretty easy from Phuket. Also check out Koh Tao which is full of divers, dive shops, etc.
ko tao and ko phi phi are really nice places to go diving i could not tell wich one is better
Would deffo say Koh Tao
The two main diving centers are the Phuket area & Koh Tao area. I would make my decision on visibility. Depending on when you want to visit, check which side has the better weather & water clarity, then make a decision.
Go to Koh Tao, go to bans dive resort, ask for steef wessal, he a Dutch dive master (say Sean sent you), go out on their boat, always a big laugh. I lived there 2 months, wicked island good night life also. I would stay over the other side of the island quit at night and still enough bars and eats, try Buddha view BBQ good tucker, although koh Tao cabana is very nice. 3000/4000 BHT a night though. You can find real cheep accommodation.