PADI list 7 dive centers in the town area of San Pedro. No others are approved, though of course this doesn't necessarily mean they aren't any good. The two marked are 5*, which means they meet certain additional standards, such as having air quality checked by PADI every 3 months:-
Ambergris Divers,
Belize Ocean Divers,
Aqua Dives,
Amigos del Mar,
Grumpy & Happy,
Protech Belize (5*IDC center),
Ramon's (5* IDC center)