Unforunately, the ferry times are such that taking a ferry to Tortola from St. Thomas is impossible during a stop on the Magic. By the time you arrived, you basically have to turn around and head back to St. Thomas to ensure you'd make the ship. The ship departs St. Thomas rather early, 4:45, so going to any other island other than St. John is out. St. John is beautiful and there are ferries every hour on the hour to get there.