Hi,
diving in thailand is mostly combined with a (long distance) speed boat trip (combined to the red sea or the maledives).
Most good places are well known to other tourists/divers, so you might not be alone when going deep.
Anyway, there are some really nice places around phi phi and a lot of other divers too.
Similan islands is spectacular, but quite far away.
In my opinion if you really want to dive, phuket is not the right place. Apart from that it is big party or simply like paradise.
cheers, enjoy your time