The Port Canaveral terminal opens st 10:30am. While it is possible to be denied entry if you arrive before your PAT, I haven’t read any recent reports of that occurring (unless debarkation is delayed for the prior cruise). Select an early PAT during online checkin and you should be good. Boarding number is directly related to PAT, so you will be waiting around if you arrive considerably earlier than your PAT.
Enjoy your cruise!