Every one has to leave the ship before the next group can board, so they won't allow you to stay on the ship or leave your luggage. I don't know about Miami, but some ports have a place for you to leave your luggage while you do some sightseeing.
Our Travel Agency (In the Netherlands though) offered us a shuttle from the pier to the airport.
Once we got off the ship we were able to put our luggage inside the bus, which dropped us at Bayside and picked us up at 7 PM to get to the airport afterwards.
This way we didn't have to carry our luggage all day inside Miami.
Sailed on :
La Palma, Amerikanis, Southern Cross, Azur (2),
Bolero, Calypso (2), Carnival Victory