Liz,
I am a certified scuba diver and have dove in Tortola, St. Barts and Barbados as well as many other Caribbean ports. The Wreck of the Rone in Tortola is one of the most famous wrecks. It was featured in the movie "The Deep" with Jacqueline Bissett. It is a very interesting dive and not hard at all. I have dove several times in St. Bart's and the water is wonderfully warm and usually quite clear. I did not enjoy Barbados as much as the other two. I am not aware of any beginning dives in Tortolla but there are beginning dives in St. Barts. You might want to consider getting certified at home before you go. You probably will need to book your own dives. I doubt the ship will have much to offer. If you are interested in shops in Tortola, St. Barts, or Barbados let me know and I will look them up in my dive book. I believe I even have their email addresses. Sounds like you will have fun!
Susan