When is your cruise? Online check-in should be available at 75 days prior to embarkation for new cruisers. I have heard that there may be a delay for the system to recognize a payment if your reservation wasn't paid prior. Online access is removed for all passengers 3 days prior to embarkation. If your cruise is very soon, yes you can show up at port and take care of the same formalities as done online for check-in. You would need to board and go to the proper locations, though, for booking things like excursions and adult dining as I don't believe they will do that at the terminal during check-in. While I haven't read reports of guests being held outside the Miami terminal due to PAT issues, your boarding number is tied directly to your PAT; so without a PAT you likely will not board early regardless of when you show up. I'd suggest arriving early afternoon, maybe 12:30-1:00pm to avoid a long wait for boarding.
Enjoy your cruise!