Closed Loop Cruises - from www.Travel.State.gov
From www.Travel.State.gov re: do I need a passport to travel:
Do I Need A Passport?
Traveling by Sea?
*"Closed-Loop" Cruises: If you are a U.S. citizen, and you board a cruise ship at a U.S. port, travel only within the Western Hemisphere, and return to the same U.S. port on the same ship, you may present government-issued photo identification, along with proof of citizenship (an original or certified copy of your birth certificate, a Consular report of Birth Abroad, or a Certificate of Naturalization). Please be aware that you may still be required to present a passport to enter the foreign countries your cruise ship is visiting. Check with your cruise line to ensure you have the appropriate documents.
Closed-Loop example: Round Trip cruise from Miami that starts and ends in the same US port - Miami.
U.S. citizen children under age 16 arriving by sea from Canada, Mexico, Bermuda, or the Caribbean may present an original or certified copy of his or her birth certificate, Consular Report of Birth Abroad, or Certificate of Citizenship.
Non- Closed-Loop Cruises: (example Starting in FL and ending in Spain)
All adult U.S. citizens arriving by sea from Canada, Mexico, Bermuda, or the Caribbean must have a passport.*
All adults arriving by land from Canada or Mexico must have a passport.
U.S. citizen children under age 16 arriving by land from Canada or Mexico may present an original or certified copy of his or her birth certificate, Consular Report of Birth Abroad, or Certificate of Citizenship.
All individuals, regardless of age, must have their own passport when traveling internationally by air.