Best shore excursions in St. John/St. Thomas

[paddles]

We're going on an Eastern Caribbean cruise the first week of Dec., with kids ages 7 and 9.

Can you recommend any good shore excursions? I'm debating between a catamaran/snorkel adventure or the Coral World by Land and Sea (that's primarily in St. Thomas).

Does the ship dock near any beaches? So, if we wanted to do Coral World in the morning, would we be close enough to go the beach in the afternoon?

thanks!!