*I checked with Sally and determined that she wanted to rent the car for a day and take it from San Juan, Puerto Rico at the airport location to Vieques but just for the day. The car would be returned back to the airport within 24 hours. Policies are specific to location and to companies so it was important to get specifics in order to be able to research properly!
If you have a rental car in Puerto Rico, check with the car rental company, most car rental agencies will not allow you to take the car off the main island of Puerto Rico. (company links open in a new window)
I first checked with anAvis
reservation and could find no info (you would have to call the San Juan Avis location to determine their policy). 1-787-253-5926
listed no restrictions either... you would have to contact the San Juan Airport location to determine availability for ferry travel 1-787-791-1805
with a San Juan reservation ... under rental terms was "Cross border traffic is not allowed. Cars are not allowed on ferries."
will not allow travel on the ferry.. their qualifications listed "Hertz vehicles must remain on the island."
nor National have a location at the San Juan Airport.
Sorry I don't have better news, but I do appreciate your question.