A rental car, as Ray suggested, is not a bad idea. BUT.. rent from an agency with a drop off point at a hotel in the pier area. Otherwise, you have to return to the airport, and shuttle back. Not that difficult, just time consuming.
I know Enterprise has a location about 5 minutes from the Miami pier, and I believe Budget is located in the Biscaynne Hyatt.
They won't let you turn in a car at the Hyatt. That's why Jose and I always rent from Enterprise and turn it in downtown on Biscayne Blvd (Hotel Dupont??) then take their free shuttle to the pier.
Thanks for the info I knew you all would know what to do. 6 of my family are flying in at the same time to Fll and we can't fit them all in the car,,so they'll have to make arrangements to get to the ship.