The weather is best at the Andaman Coast, main diving areas Koh Lanta (daytrips to Hin Daeng), Koh Phi Phi (daytrips to Hin Daeng, Koh Phi Phi and the Shark Point area), Krabi (daytrips to Phi Phi and the Shark Point area), Phuket (daytrips to Hin Daeng, Phi Phi, Shark Point Area, Similans and Racha Islands; liveaboards to Hin Daeng and Similans) and Khao Lak (daytrips and liveaboards to Similans).
Since also the diving is best in the Andaman Sea, I would say that is the area you have to go that time of year.
Some of the divesites can be quite challenging, like Hin Daeng, Koh Tachai, Richelieu Rock and Deep Six (all Simillans or a bit further North), but the only tricky divesites (depending on currents) I would say are Hin Daeng and Koh Tachai. So a beginner to intermediate diver has plenty of sites in the area to enjoy himself.