If your flight is, as most of them are, in the evening you could consider renting a car in Fort L and returning it to the airport at Miami. This can give you storage for you luggage and enable you to visit a couple of sights or even a shopping mall. You can get off the ship early and have an enjoyable day for a small amount, driving is easy there. I took my sat/nav, with a US programme last time and this made it a real treat and a piece of cake to find my way to awkward addresses, regards. .... Neil.
Its around 30 miles.
Prebooked cabs cost around $70 excl tips and tolls. Hailed cabs will be more. A cheaper version is to do a shared shuttle bus - the Tri county airport express is around $20pp.
On the two occasions that we have sailed from Fort Lauderdale transfers were included. Fly into Miami - transfer by coach to hotel in either Miami of FL - next day transfer from hotel to ship - transfer from ship to Miami airport. All transfers included in the cruise price. It's worth making an enquiry with your booking agent.
If you are doing your own thing with flights it is usual still an option to get the transfers from the cruise company, but if they are doing your flights easier to go with their transfers as well. Some cruise companies offer a night in Miami or Fort Lauderdale before transferring to the ship the next day and we go for this it is much more relaxing than off a 9 hour flight and a transfer to the cruise dock is. Most agree a taxi even one from the rank will cost in the region of $90 plus a tip.
You don't say which cruise line but we took a Panama Canal cruise with Princess earlier this year and after disembarking paid for a half day trip with Princess to the Everglades. At the end of the trip, we were deposited, together with our luggage, at Miami Airport. Alternatively, we could have booked a transfer with Princess. Can't remember the exact price but don't recall it being too pricey.