There are shuttles for most of the major car rental agencies right at the port. They are to the left of the terminal when you exit. Or just ask one of the porters. Just get on and they will take you to the car rental agency. You can then return it to the airport drop off. It is only a five minute ride.
Just remember to make your reservation prior to your cruise. This will save time and assure that there is a car available.
__________________ Cruisemates Community Leader/Moderator
"There is a great difference between being well traveled and just having been to many places." ~Me