I'm a little surprised no one's mentioned Ramon's in San Pedro (apologies if someone did and I missed it). They are a pretty decent resort, and have a very turn key dive operation. I've dove with three shops in San Pedro, and while I don't plan to go back there any time soon, if I did, would probably just stay at Ramon's if for no other reason than ease...
There are so many other places to stay that are betyer value for the money than Ramon's.
My comment is with regard to the hotel only. The dive operation is good enough.
Check out Island Divers, contact Rudy the owner.
Also, Chuck and Robbie's dive op. Owner Robbie.
I still say that Amigos for the Blue Hole 3 dive day trip and/or their Turneffe 3 dive day trip is the best way to go.