In St. Thomas, Magen's Bay was voted as one of the top prettiest beaches in the world by the Travel Channel. And while it is very pretty, it's not good for snorkeling. Reason being is all the sand - fish prefer rocks and coral as it provides food and shelter. So, if you want snorkeling, you need to go to Coki Beach or better yet, take a water taxi over to St. John and visit Trunk Bay - it's wonderful for snorkeling.
As for getting there, just take a taxi. There are tons of them around and they will negotiate prices with you. Once you've agreed to a price, they'll take you there and come back to pick you up. You don't pay until after they bring you back.
As for St. Maarten, our favorite is Orient Beach. Beautiful beach. Just be aware this is on the French side of the island, so you will see topless. And if you go beyond the stone jetty, it's clothing optional (and yes, there are signs warning you of this).
As for how to get there, we always rent a car. There are plenty of car rental kiosks at the pier. As you get off the pier, turn right and you'll see them. It's easy to get around the island, safe, and enjoyable. Definitely stop in Marigot and do some shopping at the open air flea market. But cars will run you about $45 - $75 depending on the time of year.
But you do not need to do any pre-purchased shore excursions on either of these islands. You can do everything you want to do on your own and enjoy it alot more.
Hope this helps.