Right there by the dive shop are boats for rent, from inflatables on up. I'd avoid diving from the beach there, as the boat traffic would be too risky, and there isn't squat to see in the harbor anyway. Now if you can get there for the 4th of July week carnival and fireworks, ... That's worth staying on the surface for.
If you're going to be in St Thomas/StJohn, do the wreck of the Rhone,... definately a got the cap/got the Tshirt dive. It's actually in the BVI, but the USVI DO's go there too.
Enjoy your visit, I came here from Houston a few years back, and stayed.
Darlene