St Thomas is a beautiful port and it's easy to see why so many shops there. The downtown area with shopping can be quite crowded. Shopping includes the typical diamonds,jewelry, booze, etc. If you want local arts and crafts, this is not the place for you.
We took a snorkeling excursion booked with the cruise line. They tour met us at the pier and then walked us way down the beach to a dock to board the boat. They take you out to a small island a few miles offshore with a shipwreck under the water. Think a 1970's small freighter wreck, not 1800's pirate ship wreck. The water is very clear but the wreck is deep, not shallow. It was not snorkeling along a reef as we expected. Still pretty cool though.
On the way back they provide free snacks and drinks.