We did check out Wicked Diving, and it sounds like that would have been a good fit too, but the timing of their trips is just not quite as good for us this time. (Plane tickets were booked months ago, before I discovered scuba diving, so now it's a matter of fitting in a dive trip with some of the other things we also want to do.)
One big thing is that I think I can get away with a trip of four nights maximum, since apparently Thailand has attractions outside the ocean.
Oktavia, in addition to sounding like a good fit as far as easy diving + snorkeling, has a four night trip that fits right into the beginning of our trip, which would give us maximum flexibility for the rest. During the same time period, Wicked has a three night trip, but it's to Richelieu Rock, which supposedly has more advanced diving (I'm happy to stick with easier for now), and snorkeling is not so good. They also have a five night trip to the Similans starting the same time, but a) I don't think my persuasion powers are that good, and b) even if I did, the extra night would throw us over budget.