I don't know if this would be better, but you can also rent cars in st john. Is the car rental place in st thomas near the ship or near the ferry?
We rented a car in St John and had a great time exploring the island. But we were on a Windjammer cruise that took us directly to St John.
If you only plan to go to trunk bay, you can also take a bus/taxi thing there. They run frequently & are waiting at the entrace/exit to the park to take people back to town. A car rental is probably cheaper with 6 people, but it might be easier to just ferry over and get a ride. It's not far.