We're using the "Cruise Connection" bus for our December RCI cruise - booked through the cruise line @ $45 per person roundtrip - maybe Carnival offers same? If not, their website is www.cruise-connection.com
and it shows schedules, prices,etc. They do pick ups at the McDonalds by the Speedway at 6AM (!Early!), We have used them before on a Holland America cruise out of Ft Lauderdale, and they did a great job - be warned though, that they do make stops to pick up other passengers on the way down so it's not the fastest!