According to the U.S. State Department, "as early as January 1, 2008, passengers heading to Canada, Mexico, Central and South America, the Caribbean and Bermuda by land or sea must have a valid passport."
The State Department's Web site does note that the new requirements -- for air or land -- do not apply to U.S. citizens who are traveling to or from a U.S. territory. In case your history fails you, this would include places like the U.S. Virgin Islands, Puerto Rico, Guam, American Samoa, Swains Island, and the Commonwealth of the Northern Mariana Islands. The requirements would, however, apply to cruise travelers whose voyages called at a U.S. territory on an itinerary that also included port(s) outside the auspices of the U.S.
For more info, visit The Passport Center, brought to you by Cruise Critic's sister site, The Independent Traveler.
--by Carolyn Spencer Brown, Editor