One of the most popular tourist destinations in the U.S. Virgin Islands is Magens Bay, which boasts a pristine, white, sandy beach almost a mile long. Jane and I sailed a boat to the bay a couple years ago and were amazed to find only one other sailboat anchored there. No cruise ships were in St. Thomas that day, so few people were at the beach. It was wonderful.
There are many wonderful beaches in the Virgin Islands, but Magens Bay should be on your list.