Passport question: What are the passport requirements for Caribbean cruise ships?
If you are a US citizen sailing on a closed-loop (starts and finishes in the US) then the answer is no.
The question about passports is always one to have well enough in advance to avoid last minute scramble. This policy from MSC Cruises is standard with all the cruise lines.
Cruises departing from Miami : For both security and immigration purposes, each guest is responsible for identifying, securing, and presenting all required travel documents. The necessary valid travel documents, such as passports, visas, legal family documents, etc., must be presented prior to boarding the ship and available when re-entering the United States or entering any other country.
United States and Canadian Citizens must have either a valid passport or valid documents as defined by the Western Hemisphere Travel Initiative (WHTI). Visit www.getyouhome.gov for additional information on WHTI compliant documents.
United States citizens on a roundtrip cruise departing from one United States port, sailing within the Western Hemisphere and ending in the same United States port can use a government issued picture ID along with proof of citizenship in the form of an original or notarized government issued birth certificate or original Naturalization certificate in lieu of a passport.
**Please note that Government of Puerto Rico has determined that all Birth Certificates issued by Puerto Rico prior to July 1, 2010 are no longer valid; therefore another WHTI complaint document is required.
This from Royal Caribbean International (learn more)
Cruise passengers who are U.S. citizens sailing on closed-loop cruises (cruises that begin and end at the same port in the U.S.) will be able to enter or depart the country with proof of citizenship, such as a government-issued birth certificate and laminated government issued picture ID, denoting photo, name and date of birth.
For kids without a government issued picture ID
A U.S. citizen under the age of 16 will be able to present either an original, notarized or certified copy of his or her birth certificate, a Consular Report of Birth Abroad issues by DOS, or Certificate of Naturalization issued by U.S. Citizenship and Immigration Services.