If you book your post cruise excursion through the ship, they will take care of your luggage for you. Lockers are a thing of the past in airports, which I'm pleased with. Just too easy to put something in one and take off. I'm not sure about Miami, but we've run into other large airports that have luggage storage areas that charge by the size of the luggage. Good luck, and sure it will work out somehow for you, but would definitely check with the ship for the post-cruise excursions. This way, they'll get you back to the airport on time for your flight.