Miami has a set taxi fare to/from the port and the airport - $24.00. Where are you staying prior to your cruise Karen? It will probably only be around $15 to get to the port.
I disembarked the Marina in 2011 and remember it was a nightmare waiting for a cab at the terminal. They weren't sending enough over from the waiting area or something. I think I waited in line 45 mins or so. Luckily I wasn't trying to get to the airport.
