Transfers from STT to St. John are not all that bad. If you are renting a car, rent at STT and drive to Red Hook to take the car ferry (actually cheaper than the passenger ferry if you have more than 2 people). Or there are several taxi/ferry options (either out of Charlotte Amalie or Red Hook - - warning, taxis are expensive, and you will need one on each end). Perhaps the most convenient (but not always the fastest) is the private Westin boat, but it is pretty pricey.