From what I remember, the rates between shops were pretty much the same. For my money, go with Island Divers. Rudy runs a great operation (arguably the best in SP). We switched to his shop for the last couple of days and were very pleased. Tons of dive ops all along the beach so there is no need to preselect and book your diving. Spend a hour walking along the beach to talk with them.
I am assuming that you will be flying from Belize City to San Pedro. Keep close tabs on your luggage claim tickets. They are serious about checking them in SP.
Day one, hit one of the supermarkets (we liked the ones run by the Lebanese best) and get a 20l jug of water. Break it down into bottles for daily use. Four of us went through 40l in a weeks stay and saved a lot of money over buying individual bottles. There's a deposit on the jug, and we had no problem getting it returned. If memory serves, there is a supermarket (best in San Pedro) uber close to the airfield.
On your no dive day, take one of the snorkel tours. I think the spot is Hol Chan. They feed the fish there and we saw some massive groupers, several hundred pounds each.