If you do use the car barge you will not have much Dinner Time in STJ, consider the people ferry and asking if you can find a one day jeep rental.
Taxis will work but make arrangements about that Windmill excursion + pick up ahead of time.
When we want to have dinner in STJ AND take our own car ******one person takes the car back to Red Hook at the end of the day and returns to meet us in Cruz Bay by ferry for dinner.