Your ship calls at Charlotte Amalie cruise port. A private boat takes you to Tortola in about 42 min and brings you back with margin on the all-aboard time — not on a tour operator's schedule.
Shore excursions leave in groups and come back late; the public ferry does not wait for a cruise passenger, and missing the all-aboard at St Thomas means catching up with the ship at your own cost.
A private boat departs on your time, covers the 32 km in about 42 min, and the return is planned backwards from your all-aboard.