As of June 2009, the Western Hemisphere Travel Initiative (WHTI) requires all U.S. citizens traveling by sea to present one of the following: passport, trusted traveler card (NEXUS, SENTRI or FAST/EXPRES), U.S. passport card, or state-issued enhanced driver's license.
Although passports for children are recommended, children 16 and under can travel with only a U.S. birth certificate (or other proof of U.S. citizenship such as a naturalization certificate). An original birth certificate or a copy may be used.
Do not mistakenly think of the Caribbean as a single-entry destination, like Canada or Europe. The Caribbean is a composite of independent nations and territories with individual customs and entry requirements.
According to U.S. Customs and Border Protection, U.S. citizens on closed-loop cruises (cruises that begin and end at the same U.S. port) are permitted to enter or depart the country with a birth certificate and government-issued photo ID. However, passengers may still be required to present a passport to gain access to countries a cruise ship is visiting.
For a U.S. citizen traveling directly to or from a U.S. territory in the Caribbean, passport requirements do not apply.