Phuket would be my first choice. There are boats to charter
if you wanted to sail to Phi Phi Islands (you have seen them if you watched The Beach
with Leonardo DiCaprio) or James Bond Island (featured in an old James Bond film). I am not a movie
nut but the fact that they were used in the films is a testament to their beauty. We were last in Phuket in early 2007 and it has been rebuilt from the tsunami. The marinas
are protected and suffered little damage in the tsunami.
Pattaya/Jompthien Beach area would be my second choice.
A caveat: Thai waters are polluted and the pollution attracts sea snakes. I don't recommend swimming. The Thais are wonderful people and you will enjoy your holiday.