I dived with three different operators on a 3 week trip in Feb-Mar out of Phuket and Khao Lak. I dove the Rachas, Phi Phis, and Similans and all the local Phuket sites. The diving was great, all the ops very professional, warm water and amazing visibility. And it's Thailand, which equals nightlife, good/cheap food, lodging, massages, etc.

A caveat: the high water temps (86f/30c) are bleaching the coral and if it continues, the Thai government will shut down diving even more. Some sites are closed already. One of the ops I used has decided not to go to the Similans next season. They will operate elsewhere.

But, hey, I will be back in November. Like I said, it's Thailand...