Y'all are going to love the Emerald, she's a beautiful ship. If you try the specialty restaurants, be sure to try the Black and Blue onion soup in the Crown Grill, it's awesome!
There's really good snorkeling Grand Turk and pretty good snorkeling on Princess Cay. The last time we went to St. Maarten we went to the Tiki Hut excursion. It was good snorkeling for St. Maarten and the operators of the tour were fantastic. In St. Thomas we usually go over to St. John and then up to Trunk Bay. There's a self-guided snorkel tour about 50 yards from the shore. The water is so clear and calm (if the weather's good). There's great shopping in both St. Maarten and St. Thomas, if you're into that. In fact, I think St. Maarten has surpassed St. Thomas in some areas.
Check out our Caribbean Port Chat section a little further down on the main page. You can use your "find" feature in the upper left hand corner, type in your ports, and it will pull up all the posts made about them. You might find something of interest there.
Best wishes and please let us know how you enjoyed it when you return.