Kenny:
Just got back last week from a week on island with my whole family. Only got 3 days of 2 tank diving in, and 2 of those days were fairly shallow dives since I was photographing younger family members getting certified.
That said, the last day we did Mingo and Congo reefs. They were both worth while. We had also done Calf reef and the canyons there were very nice ( bring a lite to look under the ledges).
I dove with LowkeyWaterSports this trip and although they had more divers on the boat then I usually like, we never seemed crowded and the staff was VERY good- particularly Ellen and Andy.
I thought most of the "famous" snorkeling beaches were overrated or over crowded. One of the real highlights was Waterlemon Cay. If you get a chance don't miss that. Also I would be remiss if I didn't mention the one dinner we had out at Paradisio Cafe. It was great.
Rich, above, mentioned the Rhone ( which is off Salt Island in the BRITISH VI's). You can do that from St. John, but it is an all day trip. (We went over to Virgin Gorda for 4 days and did the Rhone from there). It is one of the best dives in the Caribe, and certainly the most famous wreck in the Virgins. If I can answer any other questions, feel free. Good Luck. DD