Answer:
U.S. citizens need a passport now for cruises that stop at ports in South America, Europe, Asia, Africa, Australia and Antarctica. Additionally, U.S. citizens who travel by air to the Caribbean, Canada, Mexico and Bermuda to catch their ship must have a passport.
However, if you are cruising to and from the Caribbean, Bermuda, Mexico, and Canada from a U.S. port you do not need a passport - this is deemed a "closed loop" voyage. A "closed loop" voyage or itinerary occurs when a vessel departs from a U.S. port or place and returns to the same U.S. port upon completion of the voyage. Per the WHTI Land and Sea Final Rule, travelers on "closed loop" voyages are not subject to the same documentary requirements for entry to the United States as other travelers.
If your voyage falls under the closed loop rule you only need to carry a government-issued photo ID (such as a driver's license) and a certified birth certificate (children traveling with an adult require a birth certificate as well). A certified birth certificate has a registrar's raised, embossed, impressed or multicolored seal, registrar's signature, and the date the certificate was filed with the registrar's office, which must be within one year of your birth