If you want to check your luggage - as per CCL - you have to do it 2 hours before your posted departure time - so 2 p.m. Otherwise you will have to carry your own luggage on the ship.
https://help.carnival.com/app/answers/detail/a_id/2621/~/embarkation-and-departure
I would suggest getting on earlier so you can do muster, enjoy lunch, and explore the ship.
Arrival time back in Miami is early - we have been in as early as 5 a.m., but I would say 6 a.m. is more like it. They have started calling self-debark at around 7ish, with checked luggage around 8 -8:30. Again - this is all dependent on weather, medical emergencies, customs clearance for the ship, etc. All unpredictable.