Hi john1, welcome to Cruise-Chat! We've never been adventurous enough to rent a car, I know I'd kill myself trying to drive on the other side of the road. Most of the islands have the same rental companies as the mainland. There are some on our board that have ventured out in their own vehicle so hopefully, they'll see your post. If not, I'd just do a Google search. I'm sure it will turn up all you need.
Enjoy your cruise!