My family and I are headed to Westin St. John in mid-June. Looking online, I'm trying to understand the need for a vehicle on the island...
So...earlier this year we visited both St Thomas and St John and rented a car on St Thomas for the duration. The first three nights at the the RCC St Thomas we didn't touch it, then we took the car ferry to the Westin St John where we stayed a week and used it on three days. Then we took the car ferry back and stayed at Frenchman's Cove and used it three days out of five. We liked WSJ so much that we bought two EOY weeks and are going next year and are renting a car for a week in the 'middle' (i.e. three days without, a week with, and four days without). Part of the reason for this is that weekly rates are cost effective. The advice I can give is...
Taxis are truly ubiquitous on St John. Most of them say "Westin" on the top and are they are plentiful at both the WSJ and the taxi stand at Cruz Bay. That said, they charge per person (and bag) and are open air 'buses' that seat around 15 people. Perfect for going into town (since we didn't have to find parking), buying groceries, etc. We never actually tried to go to one of the 'main' beaches (e.g. Trunk) using taxis but spoke with many people who did.
There are no national rental car agencies on St John, for next year we went with Sunshine's Jeep Rental but there are others. IMHO renting a Jeep is silly though 4 wheel drive is useful. That said...driving on St John is not for the faint of heart. Not only are you driving on the wrong side of the road but the roads are both very twisty and have steep elevation changes. More than once that I was taking the existence of the road on faith because the hood of our car blocked any view of same. Thankfully, the donkeys are tall and the goats travel in packs. I kid you not.
If you do decide to rent at car and take the car ferry to St John, I heartily recommend Paradise Rental Car. We used them this year and will once again be using them next.