I never understood this whole thing. Whats the point of the expiration date if nobody even accepts that as the expiration date? Do they really expect a plane to be up in the air for over 6 months? I think it would run out of fuel about 179 days before that. They should just make them last 6 months longer and tell everyone, ok this is really really the expiration date now.
The passport of the visitor
is valid up to the expiration, they just want to ensure that it's going to be valid for a certain period of time once you enter the country. And it totally depends on the issuing country and the country you are visiting. So when travelling to the US or the UK on a Canadian passport, the passport only needs to be valid through the period of time I would intend to stay based essentially on my ticket or my explanation to the border agent if I drive there.
What they're really concerned about is overstaying your visit. If I visit Finland, I need a passport with at least 3 months remaining on it from the day I plan to leave and I'm allowed to stay for 3 months without a visa, hence the reason for the expiry date padding. If I arrive with the intention of staying for one day and end up not leaving, I am allowed to stay for 3 months without a visa but they can insure that my passport will still be valid. I may not stay because I want to but because I have to (i.e., I get sick or injured). So the expiry is still valid and accepted, but that date may not be considered as
sufficient at the date of initial entry.
Rules are different when entering by cruiseship.
Rules
may be different. They may not. You have to check each port/country. Some countries make a complete exception on expiry or even a passport requirement if you're coming by cruise ship, some require the same as flying in, some have rules somewhere in between, some it depends on how long the cruise ship is there, some it depends on whether you've arrived from another specific country (although that's usually more about visas), some it depends if it's a closed loop versus not. Always check with each country and it's based on your passport issuing country and/or citizenship (especially if you have more than one citizenship or passport), not the - or just the - port you start from or where you live. No matter where I live, if I'm a Canadian citizen, I have to have a passport on a closed loop cruise from a US port.
Someone with a passport from a country in South America who lives in the US, for example, is going to have a different expiry (and visa) requirement on a cruise leaving from Florida and going to the UK than an American travelling on a US passport is going to have on that same cruise whether they live in the US permanently or not. And both have to look at the requirements of each and every stop along the way and possibly as it pertains to a cruise versus flying in and whether it is overnighting or stopping for the day.
It
is something to worry about in the sense that you need to
know what the requirements for your cruise are including passport expiry dates, need for passport or not, and visa requirements as the cruise line will not make arrangements for you individually (there are blanket visas that are taken care of but if you need a visa personally, they don't do that for you). And don't rely on a TA or the cruise line to tell you what you need. There are plenty of government websites that will give you that information far more reliably. It is your responsibility and the cruise line won't let you board if you don't meet them. Sometimes, the reason the cruise line says you need 6 months even when you don't is because they don't want to figure it out for each person so they take the most conservative position for their own protection. So you also need to know the cruise line's requirements.
That's why we try to keep our passports with at least 6 months on them. Most places have no more than that requirement and many have less so while it's not true that nobody accepts the expiry date as the expiry date, I never know where I might go in that last 6 months. Now that we finally able to get 10 year instead of 5 year passports, it's not a big deal.