I have a friend of mine who has a travel agency on Hvar Island(Hvar town). You can contact him entering his web site. His name is Luka and he is dealing with all kind of renting on Hvar island like rent a car, rent a boat, bike, scooter and many other services.
You can rent your car on island Hvar. Just go here. There is a contact form where you can get more info, telling him that Pero Milos from Croatia Nudist Beaches and scuba diving.com has recommended you.