Stephanie, you are in for a GREAT cruise! We did this ship and itinerary last April and have booked it AGAIN for next April!
In St. Maarten I suggest that you do the America's Cup Regatta. It is the top rated excursion in the Caribbean, and it deserves its title! You get to help crew one of the boats that was sailed in the real Regatta, against another boat full of cruisers. (You can only book this one through the ship.)
St. Thomas has a lot of snorkeling/scuba places to go. There are many independent operators who have trips.
Aruba - we did Eli's Eco Tour and had a fabulous time. (just snorkeled, but maybe you could SCUBA there also.)
St. Lucia. We did an island tour here. It is supposed to be a great place to snorkel and SCUBA, though. I am looking at going through Islanddiversstlucia on our next cruise.
Barbados - we did a wonderful excursion on the Silvermoon. This catamaran went out to where we snorkeled with the turtles, and then we went over a ship, and then just hung out for lunch. It was like being on the lifestyles of the rich and famous.
Here is a link to our pictures: http://community.webshots.com/user/Sheffie100
PM me if you want more info.
Have fun planning. This is a great ship, and a fabulous itinerary!