I checked in for my 7 day cruise last night. The process was slowed down because the app would not let me use my passport since it expires less than 6 months before the trip (actually 3 months). Since the cruise departs and returns to the same US port, I was able to use another identification method. I was prepared for this issue because my sister had alerted me to the possibility and I was aware that this was often an issue for international flights . I don’t know if it is actually a law or just a carrier policy - either way it seems a bit excessive. I can understand if your passport were to expire during the trip or maybe within a month of the trip.