I was on a cruise where we left port, and the boat stopped 5-10 min into the harbor to allow some people coming in on a tender board the ship. That was pretty entertaining to hear the passengers from the top deck ridicule them.

However, the main point is true, you will need a passport to board a plane, so if the unthinkable happens you should have passports. The flip side is, we always leave our passports in the room while on shore, so that is where my own logic breaks down.