I usually go to Abanks' Dive Center. It is a short walk from the tenders. Pretty decent free (it used to be) snorkeling, and the restaurant has awesome conch fritters. A wonderful view of the ship from there too. You can rent a chair or a locker if needed. They also have equipment rentals.
is a map of the location. You will also see a "Diver Down" flag flying over the location from the ship.