Welcome aboard, Rebecca! Setting sail on the Disney Fantasy next year will be so magical for your whole family! I looked at the itineraries for May of 2015 on the Fantasy, and none of the three Eastern itineraries seems to include St. Lucia. So, I'm guessing you're either on Itinerary A with ports of call in St. Maarten, San Juan and Castaway Cay or on itinerary A with ports of call in St. Thomas/St. John, St. Maarten and Castaway Cay.
If you are on Itinerary A, check out the
Tortola Dolphin Encounter in St. Thomas/St. John. I don't currently see any dolphin encounters available in St. Maarten, but keep in mind that some adventures will be updated and changed each year. Since you don't sail for another year, and won't be able to log into your reservations to being making Port Adventure choices until 75 days prior to sailing (for first time cruisers with Disney) or 90 or more days prior to sailing (for those who have sailed with Disney before), you still have time!
Keep an eye on Disney Cruise Line's webpage as it gets closer to the time when you can book these adventures for the most up-to-date list of Port Adventures available for your particular sailing! Have fun planning, Rebecca!