I agree with lalenin, you should try to avoid public transit as it’s pretty much useless in Miami. A cab is probably your best option. However, if you are going on a cruise, you might want to check with the cruise line as they usually have shuttles to and from the airport. For example, Carnivaloffers transfers for $16 (per person) for a one way trip from the airport to the cruise terminal. There are also companies that have express shuttles but they appear to be more expensive than a cab as they charge per person + a 9% fuel surcharge.
Check with your cruise line for trans from airport to cruise terminal. Or catch a cab. No other way. The best part of your trip will be when you walk out of the air-conditioned airport terminal into 95% humidity and 25% auto exhaust…