Hi! Yes, I did this~ I booked Disney Ground Transportation from the Miami airport to the Miami Hyatt Centric Brickell the day before the cruise and then from the hotel to the cruise port on the day of the cruise. It is super easy to do. There was a bus full of families going. Like the pp mentioned, there was a
DCL representative the day before the cruise in the afternoon to give information and of course the morning of the cruise. We did not have our bags taken from our room, we had to bring them to the lobby and out to the bus and the bus driver loaded them for us. When we got off the bus, the porters were right there taking the bags. (At least that was my experience~)
It was only a 15-20 ride to the port from the hotel and the energy was fun!
I wanted to share my experience because there really isn't much information out there about how to do this in Miami. They just tell you they'll let you know, but there isn't much communication ahead of time.
Hope that helps a little! Let me know if you have any other questions!