They'll let them fly, especially if it is on chartered flights which is what I assume will happen. I'd like to think Carnival knows who doesn't have a passport and has notified the authorities of the unusual situation and received permission.
However those without passports will get special screening. I think they make them hop on one foot while singing the national anthem and then ask who won the World Series last year.
But the bottom line is those cruisers who continue to refuse getting a passport are being hardheaded.