Since you only have a few hours, I would suggest doing it through the cruise line. That way, you don't have to worry about what time you get off the ship, or your luggage, etc. The Miami/Ft. Lauderdale area is spread up and down the coast. It can take a chunk of time just getting around if you don't know that area. Or, at least choose something not too far from the airport, like Los Olas Blvd or the water taxi.