I did a similar trip last year for the month of December. Dived with Santana out of Phuket for my liveaboard. They are really good value, run by a really experienced German couple, its a quite basic boat but run with really good humour! did 4 nites and 5 days of diving which took us as far as the Similan Islands but not to Burma. The diving was out of this world!
Stayed in Koh Lanta as well - dived with a great outfit called Scubafish. they are in the north of the island - cant quite remember exactly where.
Spent a week on Koh Phangan - diving not so good there - bad vis altho close encounter with a bull shark!
You will love it out there.
Good luck!
