The best time for liveaboards to the Similans I think is April/May. Especially in April there is still plenty of boats to choose from, and it is quieter with divers than in January-March, and also the weather is still very nice. For Burma I would recommend the same period, but the best choice of boats you get a bit earlier in the year, March.
Regarding hhotels: really plenty of choice, it depends on your budget and wishes. There's hotels here from 300 Baht per night up to 10000 Baht, and that is only in Kata.