Requirements to enter Mexico:
Effective March 1, 2010 U.S. citizens traveling to Mexico as tourists must present a valid passport or passport card to the immigration authority at the point of entry to Mexico.
U.S. citizens living in the border area with Mexico may cross the border without these documents. Nevertheless, they should present them if they are traveling 20 miles or more, beyond the border into Mexico, in the designated checkpoints.
• U.S. citizens entering Mexico by sea may enter without such documents if they are visiting port towns for a short visit. They should present a valid passport or passport card if they travel beyond these port towns.
http://embamex.sre.gob.mx/usa/index.php/consular-services ..
Requirements to re-enter the United States from a closed-loop cruise:
U.S. Citizens on closed-loop cruises will be able to enter or depart the country on the cruise with proof of citizenship, such as an original or copy of his or her birth certificate (issued by the Vital Records Department in the state where he or she was born) and, if 16 or older, a government issued photo ID. If the child is a newborn and the actual birth certificate has not arrived from the Vital Records Department, we will accept a Hospital issued birth certificate. The United States does not require you to have a passport. (A Consular report of Birth Abroad issued by the Department of State or a Certificate of Naturalization is also acceptable.)
HOWEVER, it is possible that one or more of the Caribbean Islands on your itinerary, does require you to have a passport to enter their country. In that case, it is very possible that the cruise line will require you to have a passport to board, even if it is not a U.S. requirement. You should always check with your cruise ship, travel agent and or destination country to confirm the requirements for entry into the foreign countries you will be visiting.
https://help.cbp.gov/app/answers/detail/a_id/1139/kw/%22closed%20loop%22 ..
If your cruise begins and ends in the same U.s. port, you do NOT need a passport. Many people do not know about this and give incorrect info without references. Your cruise line can confirm this info. See site below and read section on closed loop cruises.
You need a passport. Up until a few years ago you could travel to Canada, the Bahamas and Mexico with just a birth certificate and state ID but the laws have changed. You must have a passport to leave US territory.
Charlie is an idiot. You need a passport.