Hi! You're going to love Thailand. I did.
Bring a fish lense and a wide angle, for sure.
I was just on Black Manta (1 week) and Pawara (2 weeks).
Both are quite nice. Pawara is aiming at the Ocean Rover/ Philkade market, but prices still midrange this year.
Both boats share the distinct disadvantage of no photo facilities. At all. No camera bucket, no camera table, no cubbies, nuthin. They have desks in the lounge for using your laptop though. Luckily I knew before I went and arranged a big rubbermaid tub to be my camera's "house" and "designated" one of teh two rinse buckets as a "no mask" zone (and people mostly respected that). On both boats I was the only person with a camera bigger than a P&S.
Pawara was great about letting me go do my own thing - once they were satisfied I was a good diver, they let me stray a bit from the dive group. Talk to Duke in advance if you plan on being like that cause they mostly get follow-me divers.
Oh, and a side note, Black Manta has British (!) electrical plugs. Adapters were in short supply.
Americans seem to really like the Philikade, surprised nobody has mentioned it, although it is pricey I hear.
I usually book through John Williams at Siam Dive & Sail. He knows the various boats pretty well and can advise you.
P.S. I heard Jazz has cockroaches but is otherwise quite nice.
:-)
ocean rover or one of the mermaid boats for me...book before you go or you could end up on something you might not like.