I did my first (resort course) dives in 1989 on St. Johns. As most first dives are, it was awe inspiring. Water clarity was superb, (100+ ft) and the corals appeared healthy. The scuba bug bit me hard as a result of these dives, and I immediately got my OW certification. Having subsequently been to the Caymans, BVI, Coz, GBR and other places, St. Johns has average diving IMHO. However for a combination of top side natural beauty and good quality (if not world class) diving, it's hard to beat the Virgin Islands, either US or British. If you want to see the best diving in the Carribean, go to Little Cayman or Coz (I hear Turks and Caicos rounds out the top 3 - I'm going to Provo in July, but haven't been yet). If hiking, beaches and other non-diving activities in a not over-developed area is as important as diving, I'd say go with St. Johns.
Rich