Budget, Avis and Payless have a shuttle to cruise port from airport rental.
They pick only people with reservations for these companies. and absolutely refuse people from other companies. Actually the security at port exit send you to the taxi stand for other rental companies.
Shuttle runs every 40 minutes or so and last shuttle from airport to cruise port is at 4PM
Also, if you have more than 2 people in your party, they don't allow more than 2 on the shuttle in any direction. You will have to pick-up the other people from cruise port after getting the car or drop them off before returning the rental.
If the shuttle van is not full, you may have a chance to get the other people on.