we did the excursion to trunk bay on our own as well, not with a rental car, but we took the taxi to town and took the ferry over to st. john, and then taxi to trunk bay, on the way back we took the ferry to red hook and taxi back to the ship. it was very easy to do. at trunk bay there are taxis waiting to take you back any time you want to go. the time to and from the ship was about the same either way, it just depends on whether you want more time on the water or land, and you can choose based on how soon you want to leave as ferries run on different schedules depending on which destination you want. like i said just choose what time you want to leave for st thomas and choose a ferry based on that. most of the taxis will hold mutiple passengers and you might have to stop a time or two, we stopped at two ships before going back to the magic, but it wasn't really a problem, we got back in plenty of time to relax on our verandah and watch the jellyfish in the harbor and then watch departure. it was very enjoyable and easy to do. the taxi driver to trunk bay even stopped at a bank in order for me to get some cash.