In St. Thomas, the taxis are waiting right outside the security gate on the dock...there will be a representative waiting there to direct you to a taxi (they have open air buses that hold maybe 20 people, so they fill them up for each destination). We cruised in June to the eastern Caribbean and took a taxi to Magens Bay Beach (taxi ride was $8/person one way) and admission fee to Magens Bay was $4/person. The beach is maybe ten to 15 minutes from the dock and is gorgeous....soft, clean, sand...beautiful water...lounge chair rentals available $5...bar and snacks right on the beach...restrooms, etc. The taxi will drop you off right at the beach, very little walking. It was beautiful. If you snorkel, there isn't much to see at Magens Bay...perhaps Coki Beach would be the place for you, I heard there is good snorkeling there. We also went to Sapphire Beach in St. Thomas..nice, quiet beach but nothing much there....a small gift store that sells food, but you can rent chairs here also and there is a restroom. My recommendation would be Magens Bay if you want to just relax, have lunch and drinks on the beach, heavenly.
In St. Maarten, the taxis are at the end of the pier...we took a taxi to Orient Beach...WOW! A stunning beach with clean, beautiful water and all the water sport activities you could ask for...jet skis, flying fish (like a banana boat ride), etc. There are lots of restaurants and bars, lounge chair rentals, waiters walking the beach taking drink orders. In certain areas you will see the occasional nude sunbather but we didn't see that many when we were there. We asked our taxi driver to pick us up at a designated time and he was there right on time. If you like to snorkel, I heard that Pinel Island is a great spot, you can see the island from Orient Beach. It is accessible by a short ferry ride.