Well, it took some searching, but I finally
found this on the Carnival website (why do
they "hide" these things
U.S. citizens must present proof of citizenship in the form of a passport (valid or expired for less than 10 years), original birth certificate, state-issued certified copy of a birth certificate from the Department of Health and Vital Statistics, or original Naturalization papers.
So, what Luanne said is true (not that I'd ever
doubt her info
You'll be fine with your expired passport.
Have a wonderful cruise!