Check out the following on the Dept of State website:
http://travel.state.gov/travel/cis_pa_tw/cis/cis_970.html
ENTRY/EXIT REQUIREMENTS: For the latest entry requirements, visit the Embassy of Mexico’s website or contact the Embassy of Mexico at 1911 Pennsylvania Avenue NW, Washington, DC 20006, telephone (202) 736-1600, or any Mexican consulate in the United States for the most current information.
Since March 1, 2010, all U.S. citizens – including children – have been required to present a valid passport or passport card for travel beyond the “border zone” into the interior of Mexico. The “border zone” is generally defined as an area within 20 to 30 kilometers of the border with the U.S., depending on the location. Regardless of the destination in Mexico, however, all U.S. citizens age 16 or older must present a valid U.S. passport or passport card to re-enter the U.S., even by land.
All Americans traveling outside of the United States by air, land or sea (except closed-loop cruises) are required to present a Western Hemisphere Travel Initiative (WHTI) compliant document such as a passport or a passport card. While passport cards and enhanced driver’s licenses are sufficient for re-entry into the United States by land or sea, they may not be accepted by the particular country you plan to visit; please be sure to check with your cruise line and countries of destination for any foreign entry requirements. U.S. legal permanent residents in possession of their I-551 Permanent Resident card may board flights to the United States from Mexico.
It's important to note that this cruise falls within the exceptions. I called
DCL and they explained that U.S. customs makes exceptions to the passport requirement for cruises which depart and return from a U.S. port (a "closed-loop" cruise). In these cases, you can travel with a valid driver's license and a certified birth certificate showing U.S. citizenship (these are usually embossed with a seal and copies are not accepted).
I took this directly off the
Disney Cruise Line website today:
Travel Documentation for Mexican Riviera Cruises
The following guidelines are based on U.S. government regulations, which are subject to change at any time. It is your responsibility to check with your
Travel Agent, government agencies or the consulate in your home country to ensure that you have all the proper documentation to board the ship and enter each of the countries on the ship's itinerary. If you're visiting any country other than the United States, remember to contact the consulate of that country to ensure you have the documentation required to visit. Travelers can visit the U.S. State Department's website at
http://travel.state.gov.
Required Travel Documentation
Select your citizenship to read details about required travel documentation:
United States Citizens
Canadian Citizens
Non-U.S. Citizens
United States Citizen
All Guests claiming U.S. citizenship, including children, must present one of the following documents as proof of U.S. citizenship for each member of their group:
A valid, unexpired U.S. Passport
A valid, unexpired U.S. Passport Card
A valid, unexpired state-issued Enhanced Driver's License (where available)
Trusted Traveler Program Card (NEXUS, SENTRI, FAST)
U.S. Citizens 18 years and older will be able to depart and enter the country with an original or certified copy of their birth certificate and government-issued photo ID.
Children under the age of 16 may present an original or certified copy of their birth certificate, a Consular Report of Birth Abroad, or a Naturalization Certificate.
Photocopies of any of the above documents are not acceptable.