In Georgetown, near the main harbor, is the Atlantis shop on the water.  They take you out (on a boat) to their submarine docking station just offshore. For about $70.00 (the cost of a two-tank dive) you get to go in a state-of-the-art submarine down to about 100 feet.  They have a truly professional crew of Australian/Canadian/Americans that operate these around the world.  Very safe tour.  If your group won’t do it…there is the Nautilus semi-submersible just up the street.  It, however, never submerges. It is like the Disneyland submarine, but larger, with drink service. Whichever you choose, the evening tours are the best, as the predators/nocturnals start to come out (Tarpon, barracuda, sharks, squid, lobster, and octopus). 949-7700 (Atlantis); 945-1355 (Nautilus)