Yep, right out of the State Department's website; Some countries require that a travelerís U.S. passport be valid at least six months or longer beyond the dates of the trip.
Of course, since they don't list the countries, everyone has adopted the policy to be on the safe side. In fact, here's what RCCL says about it;
Royal Caribbean International strongly recommends that all guests travel with a passport that is valid for at least 6 months beyond the end of the cruise.
So, while it's not required, it is highly recommended if traveling outside the country as it's better to be safe than sorry.
And another thing that alot of people aren't aware of is they should register their trip whenever traveling overseas. You can receive free travel updates about where you're going as well as other information. It's called STEP (Smart Traveler Enrollment Program); https://step.state.gov/step/