Traveling with a valid passport isn't just a good way to go. It's the recommended way to go.
FINAL Passport Rule Issued for Land and Sea Borders
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.
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 driver’s 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.
The good news for a majority of cruise passengers – American citizens that leave and return on their cruise from the same U.S. port – is that the travel document requirements will remain largely unchanged from how the industry is operating today.
CLIA continues to encourage travelers to obtain passports to allow the broadest travel opportunities. Your clients can visit www.travel.state.gov to obtain information on how to apply at more than 9,000 passport acceptance facilities in the United States.
http://www.travel.state.gov/passport/passport_1738.html or call the National Passport Information Center toll free at 1-877-487-2778. Foreigh Natioanls should contavt their respective governments to obtain details regarding current passport requirements and application procedures.
