We've sailed out of Miami twice and flown into FLL both times. We've done it two different ways. Hopefully you can learn from my mistakes!
The first time I used Hotwire and let it pick me any hotel downtown Miami for $89. We ended up at the Intercontinental (a steal for $89) and had a nice stay. We used SAS transportation from FLL to the hotel which does have boosters *sometimes* but not guaranteed. We brought a Bubble Bum for our booster-using child. The morning of the cruise we took a cab (~10 minutes) to the port. Again, I don't have a kid in an actual 5-point seat so we just used the Bubble Bum. On the way back we used SAS again and it was insane. They didn't have any car seats, there were unrestrained toddlers in the van and the driver was a maniac (I've ridden in taxis all over the world: Rome and NYC are particular nuts and this guy was ON PAR). I will never use SAS again after that experience.
The second time we had my mom and step-dad along so transport was getting more expensive. We ended up renting a van. We could have reserved a booster from them (or car seat, if we had a smaller kiddo that needed it). We drove the van to a hotel in Fort Lauderdale (from FLL airport) and stayed on the beach. That was a big mistake - spring break. So, never again with that! The next morning we drove that van to the port. My step dad dropped everyone and luggage off and we all went inside and waited while he returned the car. The
Avis return is in downtown Miami and they have a shuttle. That went quite smoothly and I would choose that option again, just not stay at the beach in Fort Lauderdale. At the end of the cruise we did it in reverse: shuttle to Avis, picked up van and then since we had a later flight we went and did something else with the car before dropping it off at the airport. My signature has links to trip reports for both vacations if you're curious to read more details of our adventures. Both were 5-nights: one in spring and one in fall.