I have flown into FLL for a cruise out of Miami a few times. The first time, I flew in the day before (because the day of the cruise the flight was too expensive). I rented a Car at FLL, and I was able to drop the car at MIA with no drop charge.
The second time I flew into FLL the day of the cruise, and flew home the day after. I Purchased a Carnival Transfer at the airport, then I rented a Car at the Miami airport, and dropped it at FLL the next day.
I find it much more fun and less stressful to fly before and after.
My next cruise is in January on the Dream. There will be 7 of us. We will fly the day before and after. My Father-in-law accumulated 400,000 Wyndham reward points while he was working for 2 years as a consulting Engineer in China (that is enough for 30 days in one of their best hotels), so the hotels will be no charge. I searched a few companies and the cost of Transportation for all 7 is about the same as a 24 hour mini van rental, so we will rent our own mini van each way.