Hi Newbie! You've recieved some wonderful advice from some very experienced cruisers!
I really wouldn't worry about motion sickness. You can get the patches, but if you don't, ginger, bonine, and green apples work great. Also, a Sprite or 7Up with bitters works wonders, too.
I totally agree with Trunk Bay, it is one of the top 10 beaches in the world. However, if you want to stay on St. Thomas, Magen's Bay or Coki Beach are two of the best for ammenities, great water, and nice beaches. The snorkeling in Coki is pretty good, too. The snorkeling at Sapphire Beach is much better, but the beach itself is not as nice. There are tons of little rocks along the shore. Trunk Bay is just magnificent. There is a self-guided snorkeling trail about 50 yards from the beach. It is a bit of a journey, but well worth it.
St. Maarten has tons of nice beaches. Orient is a clothing optional beach, Maho is where the jets land nearly on top of you, Pinel Bay is nice too. The last time we were there we did a snorkling trip through the ship to the Tiki Hut. It wasn't a beach, but a nice set up on a floating dock. You could snorkel, swim, sit back and enjoy a drink at the bar or in your floating chair. The operators were wonderful and would take you back to the ship or to town for shopping.
Hope you have a marvelous time, and as poormom said, just be warned, cruising is addictive!