This just came in. I highly receommend getting a passport anyway.
Yesterday, the Departments of Homeland Security and of State issued the final rulemaking concerning the requirements of passports for land and sea border crossings under the Western Hemisphere Travel Initiative. After years of development following 9/11, this ruling is the end of the process.
Effective June 1, 2009, the following rules take effect for cruise passengers:
U.S. citizens on cruise voyages that begin and end at the same U.S. port (closed-loop itineraries) must show proof of citizenship* and government-issued photo ID (such as a driverís license). A passport will not be required for passengers that fall into this category.
*Documents include: Original or certified copy of birth certificate; Naturalization papers; Consular Report of Birth Abroad issued by Department of State
All other passengers and/or itineraries (such as cruises which begin in one U.S. port and return to a different U.S. port or any cruise that begins or ends in a foreign port) will require a passport or other recognized document. For a list of accepted documents, see www.travel.state.gov.