Take the sea plane! Seaborne Airlines operates between St. Thomas, St. Croix, Puerto Rico and Vieques, and is talking about expanding service to Tortola (hopefully soon!). In St. Thomas, planes depart from Seaborne's air terminal, just next door to the Charlotte Amalie ferry terminal, making it a convenient option for folks traveling from Tortola to St. Croix.
HB has been dying to try the seaplane since we moved to the BVI. I'm a little nervous, myself, as these 20-passenger Twin Otters are awfully small, but the idea of only a 20-minute ride to St. Croix is encouraging.
But if you're looking for a slightly adventurous, definitely Caribbean experience while you're in the Virgin Islands, you might give Seaborne a try.