so much depends upon what you want to do. St. Thomas if you want gorgeous beaches, scenery and really good snorkeling then you really should make the effort to get over to St. John Island, you can do it on your own and have all the time you want go to www.vinow.com
for ferry times, rates and beach information. If you want to stay on St. Thomas proper and want beach/snorkeling then i would grab a cab to Sapphire Beach, if snorkeling doesn't interest you at all then grab a cab to Megans Bay Beach. After your beach time you can go shopping if you like, if you dock at Havensight there is a nice mall right there, another fun thing to do is to take the short walk from Havensight to the Paradise Point Tramway, the view from up there is breathtaking. If you just want to tour the island and make a short beach stop then hire a local cabbie for a few hours, negotiate a round trip and pay when you are returned to the ship.
St. Maarten - Orient Bay Beach is by far the most popular beach on St. Maarten it is french so it is optional topless and there is a clothing optional section that is aways from the mainbeach and it is clearly marked, this is a beautiful beach, grab a cab at the pier. IF you want a fun excursion for the family i would suggest you book thru the cruise line the Golden Eagle Catamaran sail, we totally enjoyed this tour - you sail to an island preserve where you can swim, snorkel or just veg on the beach, food and drink are included. Shopping is also very good in St. Maarten, after your tour grab the water taxi at the pier- for about $7 you buy an all day fare you can come and go as you please and the water taxi puts you into downtown- this is on the dutch side. If you want to shop on the french side you will need to grab a taxi. If you want to just tour the island then hire a cab for a few hours.