This is not really your scenario, as you will be leaving the car at the port.
But after seeing the topic can't help but say a few words. Miami Airport is the Worst place I have ever returned a rental car.
Not once but twice I have gotten lost trying to return cars and make flights. I almost left a Budget Ford Taurus at a gas station once and called a taxi. I only speak english.
If I am going to take a post cruise 3 day extra trip, with an Airport Rental Car, it won't be in Miami