Couple of questions as this is my first time at this property...
EDITED...with thanks to
@Eric B
If you are staying at the WSJ, renting a car is optional. We had a car the whole time on our first visit to St John (we picked it up and dropped it off at STT because we were also staying on St Thomas) and only used it for visiting the east side of the island (e.g. Coral Bay). Next time we are staying for two weeks and have a car for a week in the middle.
Taxis (really 15-ish passenger open air buses) are more or less ubiquitous on the west side of the island and are usually parked at the WSJ and in Cruz Bay waiting for passengers. The service from the major beaches on the west side of the island is pretty good as well. They do charge per person though...so if you are 6-9 people renting a minivan or SUV might be a good choice.
There is a dedicated ferry service direct to the Westin (if you are staying there) that charges a flat fee for the length of your stay. Good if you intend to go back and forth, expensive otherwise. There are also public ferries that go between Red Hook or Charlotte Amalie on St Thomas and Cruz Bay on St John which are quite inexpensive.
Lastly, there are car barges that travel between Red Hook and Cruz Bay. Depending on whether or not you want a vehicle for your entire stay on St John, it can be less expensive to rent a vehicle at/near STT and take it across to St John.
We've done both the car barge and the passenger ferries out of Red Hook and Charlotte Amalie. But we are planning on taking the Westin ferry next year for various reasons.
Virgin Islands Ferry Schedules Between (St. Thomas - St. John) (Water Island) (Frenchmans Reef) (Tortola) (Virgin Gorda) (Jost Van Dyke) (Anegada)
www.vinow.com