St Thomas I recommend Saphire as being beautiful, quiet, good snorkeling, amenities and gorgeous view.
St John, any public beach. Most popular , Trunk Bay.
Barbados , any Beach on the south shore.
St Martin Dawn Beach has everything you are looking for.
Tortola. Most popular will be Cane Garden Bay. Speak to a cab driver about other choices like Apple, Smugglers or Brewers. Be sure to make a round trip arragement if you go somewhere besides Cane Garden Bay. The suggestion of the excursion to the Baths and Devils Bay on Virgin Gorda is a good one.