Although there are no Dollar depots physically located at the ports (Fort Lauderdale, Miami and Tampa), there are free shuttle busses that run regularly between the port and the local Airport depot. In general, the busses run every 15 minutes, although there is a courtesy phone you can use to call the depot if you have just missed a bus.
When you arrive at the Port, simply follow the car rental signs and use the free bus to take you to the airport. On your return journey, if you are departing on a cruise, simply drive to the Airport depot, leave your car, then use the free bus to take you to the Port.
On your voucher, the depot will be listed as the local Airport location that services the Port required.