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.