As a general rule, passports should have at least six months of validity when travelling internationally.
Hupla Group LTD recommends that all guests travel with a passport valid for at least six months after the return date unless noted differently.
Most countries will not permit any travellers to enter their country unless the expiry date of the passport is at least six months after the final day of travel. This blanket rule is to avoid having travellers overstaying their passport validity.
There are exceptions to this rule. For example, a US passport holder can travel to Canada & Mexico with three-month validity on their passport.
If you are returning to your home country on a direct flight, your home country will also often waive this requirement, ie a Swiss can travel back to Switzerland on an expired passport if it is a direct flight.
Don't try to negotiate with the ground staff in the cruise terminal or at the airline counter, the airline/cruise line will get heavily fined if the passenger arrives in the country of destination and is denied access to the country.