The western caribbean route is more renowned for snorkeling and water sports, the eastern itinerary more renowned for shopping. But, there are always exceptions. If you are stopping at St. Thomas, do an excursion to St. Johns which has some fabulous snorkeling. St. Maarten on the eastern route also has some great water activities like the Americas Cup Yacht Races and the Golden Eagle Out Island Catamaran Excursion. There are a lot of good beaches (not great for snorkeling, etc.) on both islands. Orient and Dawn beaches on St. Maarten (on the French side so there are clothing optional areas and they are used) and Magens Bay, Sapphire Beach and Coki Beach on St. Thomas.