There are many nice beaches and sites on St. Thomas, but our favorite excursion (and favorite sight!) when we are in St. Thomas is to go to Trunk Bay in St. John. It is mostly a nat'l. park and is georgeous! We think it is far more beautiful than the famed Magen's Bay. (Look it up on the net on USVI.com, I believe is one site). All ships offer snorkeling exursions there. They take you to Cruz Bay on St. John by ferry and then you get in an open air safari bus w/a top on it for shade. Then you are driven to Trunk Bay. Along the way you get some fantastic views and the drivers stop for picture taking opportunities. There are showers, restrooms, etc., at the beach there. The sand is powder white and water is georgeous! The snorkeling is good, however not quite as good as it USED to be due to many visitors - but like all snorkeling areas, better on some occassions than others. (If your son has not snorkeled in Hawaii, the Caymans, Belize or Cozumel, he will think it's great!) Since this excursion is offered fairly early in the a.m., you return to the ship in PLENTY of time to shop in Charlotte Amalie. Also, there is an area right by the dock called Havensite Mall. Many repeat cruisers have learned you can get all the same stuff there for about hte same price. But I'd still advise seeing the main town.
Re: St. Maarten/St. Martin - Yes, Dawn Beach is nice. There is a restaurant (that is the ONLY restroom there and you may not use it unless you have been a patron that day). There is a little hot dog type snack shack, too. You can rent snorkeling equip. for about $8 or so. The beach is nice but small. There is a small reef within easy swimming distance from shore. Now a word about Orient Beach. That is where you can parasail. There are several vendors. There are many open air, shade-covered, full fledged restaurants w/nice entrees/sandwiches to snacks to margaritas, and also restrooms in back w/rudimentary showers in some cases. Food is GOOD. Prices aren't too bad. You can rent waverunners there, & I think there was a water trampoline, and it's more lively than the other beaches, so there is more to watch. Maybe because I used to live in FL, so even though we were there during spring break, it wasn't what I'd say uncomfortably crowded. A word of caution that will no doubt probably make you decide not to visit Orient: Sometimes people refer to it as "the nudie beach". We heard this several times. We were once on the island for a wk. w/my son's baseball team and never visited this beautiful beach due to this! What a shame. There IS a famous nudist resort at the far right of the very long beach if you are facing the water. I'd say we saw maybe 2 nude people the whole afternoon we were there. One was a fat old man. One was a lady from a great distance as she was well down the beach and we were walking in that direction toward the nudist resort. There were a FEW topless women (& you may also see this at Dawn Beach, even though it IS on the Dutch side and Orient is on the french side), but it isn't as obviously noticeable as you might expect. We and some other parents took our teens there last spring and it was no big deal. 98% of the people were covered top and bottom. Your son will most likely meet up with kids on the ship and might find he wants to go wherever they are going on excursions. If they are going to a certain beach, he might want you to take him there. (Don't take a ship excursion to the beach in St. Maarten. Just hire a cab at the port. The driver will return for you. If he doesn't, others will be there waiting. No problem.)