There are 2 options for car rental.
The first is to take the ferry to St. John and have your rental car waiting there. Here are a few good options for renting on St. John:
The second option is to rent on St. Thomas and barge the car over to St. John. There are a few Rental Car Agencies that will allow you to rent in St. Thomas and barge to St. John. These are:
Remember to drive on the Left!