The main issue isn't the cruise, its the what happens when there is an emergency. In order to get off the ship (other than for a day excursion), they may require a valid passport to enter the country (to go to a hospital or catch your flight). In the Caribbean, many do not have the "6 month validity" requirement. Columbia was one exception if you are doing a Panama canal cruise. For the US, you only need a valid one to use it to re-enter. So while a guideline, if you are getting close to 6 months, best to get it current to renewed to eliminate any worry.
I would think as noted above, it would be more pertinent on a European cruise (of course, in this case, you also flew there anyway so you would have had it).