Looks like you are on the Carnival Ecstasy, is that correct?
Since you are flying in on the day of the cruise, why not get transfers through the Carnival? They do them from FLL to the port of Miami for $30 each one-way. A Carnival rep will be at the airport with a sign and they will make sure you get to the ship with your luggage.
Since your return flight is so late, you might want to look at doing a post-cruise excursion. Many of them are set up specifically for guests with later flights and they'll drop you at the airport. It certainly beats spending 8+ hours sitting at FLL, which is what you are looking at unless you rebook an earlier flight.
I am not a fan of cruise ship transfers, so my advice would be on arrival to use: SAS Transport Quality Shuttle Service at Affordable Prices. They are the most resonable @ $15.00 pp. We always use them. By the time you get your luggage and board their van, you should be arriving at the ship in time to board, eat lunch and relax,and explore the ship.
On the way back I agree with Dave, do a ships post excursion. This way the ship worries about your luggage, not you,and you get to extend your trip a bit, in plenty of time to get you to the airport, without the being in the rush of most cruisers leaving late morning /early afternoon.
Any more questions please ask, we are ready to help...oh. btw..did you get travel insurance?
Trip, with her book & tea!
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Personally I must agree with the advice of SAS Transport Quality Shuttle Service at Affordable Prices. They are great at only $15.00 per person and have always expedited getting many of my friends and myself and family to vessels. SAS recently upgraded their service to include reservations service to include Android and I phone services.