Depending on your cruise they do have a dolphin tour when you are in St. Thomas. The thing is that the tour is on Tortola in the British Virgin Islands - which is nearby. Actually they show two tours, one being 5 hours and the other being 5 and 1/2 hours, but they both appear to be the same basic tour...mostly traveling by boat and going through Immigration twice (British and then American), and 30 minutes in the water with the dolphins. Not worth the hassle in my opinion. Plus, since it is a closed loop cruise and passports aren't required, they ARE required to do that tour since you are entering another country by means other than the cruise ship.
If a person has even a little bit of acrophobia I would advise them to not do the parasailing.