To be honest we took the shuttle once, had to load and unload our own suitcases and then stopped at every terminal to pick up other passengers, we won't be doing that again. However since the bus company contracts with Carnival I am sure they would allow you on the ship just like they hold the ship for excursions that are late but personally I would fly in the night before, stay at a hotel near the airport and uber or taxi to the port the next morning