I just got back from a Caribbean cruise last week - for a Seattleite used to murky 55-degree water, Trunk Bay beach on St. John is just unbelievable. For snorkelers and divers, it has a shallow coral reef around two islands about a hundred feet offshore. National Geographic rated it the second-best beach in the world.
One drawback is that Trunk Bay gets crowded in the mornings when the cruise passengers shuttle over from St. Thomas. All the beaches on St. John are public and looked spectacular, so take your pick.
We didn't get around to Tortola or St. Croix (which is USVI if I recall correctly), but family have told me that Tortola is very nice, quite relaxed and low-key.
Have a great trip - we did!
Keynes is dead and we are living in his long run.