I was in Belize in December of 2006, and dove with ... damned if I can be sure, but if I had to take a bet, it'd be Amigos Del Mar. Fun diving in the coral canyons off of Ambergris. Honestly, it wasn't a dive vacation for me, it was a vacation that had a good amount of diving mixed in, and there seems to be enough dive shops in San Pedro that if you don't like the one you're with, settle the tab and move to the next. Of course, thats a year and a half ago now too.
Either way, and the important part that was missed from Don's original post is that Belize is awesome for diving and for everything else - even at the very end of the rainy season. I wish I was there right now.