Land at St Thomas Airport (STT), step onto a private boat, and be on St John in about 25 min. No ferry timetable, no overnight on the wrong island — the transfer is timed on your flight.
The public ferry runs on its own timetable and rarely matches an arriving flight; a missed connection means a night on St Thomas instead of St John.
A private transfer leaves when you land, takes your luggage in one go, and covers the 19 km in about 25 min.