I haven't seen a locker in an airport in years, and we've traveled through a lot of them. If you book a post-cruise excursion through the ship, they'll take care of the luggage for you. Some airports have a locker room where you can leave luggage, but they are limited as well. I'm really not sure about Miami, specifically. We had a 4 hour layover there last year, and had to carry our luggage around with us. Good luck!