It's actually very easy to do this on your own. Take a taxi to the ferry dock (Red Hook I believe) and take the ferry over to St. John. Lots of taxis and shuttle buses there or you can rent Jeeps, etc... We have done this 2 or 3 times. We just catch one of the shuttle taxis to Trunk Bay. We spend the day and then head back to the ferry. We make sure to give ourselves plenty of time to get back. If the ship leaves at 5:00 pm then I would start heading back to the ferry at about 2:00 pm. Honestly, I don't know what you would get in St John if you left at 8:30 and came back at 12:30. Not much time to see anything or even spend at the beach.