If you leave only a few hours drive for the day the ship departs you'll have more than enough time to board as the ship probably doesn't sail until 4 or 5pm. They won't let you board much earlier than noon as they have to clear the ship first. Although you've pre-registered, you'll have to go through the processing at the port but that is usually very fast. Just confirming information and issuing your sea passes (ship credit cards/room keys). The parking service (Cruise Parking of Port Canaveral) needs to be reserved on line before you get there. Don't forget to take a backpack or something like that for your off the ship activities. Happy cruising!