We were recently in the same position with an 8pm flight out of Miami. A $12 uber ride from port to Bayside was quick and we stowed our bags through a luggage service that was around $15 for the day at a luggage shop at Bayside. We booked the boat cruise and hop on hop off bus for the day. $55 each starting from Bayside. Then booked $25 uber to the airport.
Cruise was $179 pp for basically the same and glad we didn't rent a car as originally planned as parking was noted as expensive and a hassle to navigate traffic. It was nice to sit back and enjoy the ride.
Boat cruise well worth it. Hop on and off wasn't as good as we expected but got to South Beach and a big loop around town.
