There is some good info on this excersion on the Princess boards from Norm,
he highly recommends doing this, at least he really enjoyed it. Here is a little bit about what he wrote about the excersion.
I found this from a previous post about the Champagne Catamaran Sailaway to St. John, sounds like a winner:
"Be sure to book the "Champagne Catamaran Sailaway to St. John" for your port of call in St. Thomas. It's a delightful day of sailing, swimming, and snorkeling (gear and lessons provided for those who need them) that's definitely worth every cent of the price. "
Sounds like a fun trip, enjoy!