i used
Divetheworld Diving the World to book the mermaid I in feb 05. decent boat, good food, knowledgeable dive masters, nice diving, and price wasn't that bad for five days diving.
have never used them but scuba cat usually runs trips. they have a boat that stays at the similans during the high season and they use a speed boat to transfer you back and forth.
i have used dive asia and warm water divers for day trips. both good operations.