WHAT DID I SAY, DONT DRINK THE WATER!!! According to National Center For Infectious Diseases, food and waterborne diseases are the number one cause of "messed up shits" to travelers. As you see in this direct quote from the site. (Note, this is not plagiarism, I asked them very nicely, even with sugar on top.)
Food and waterborne diseases are the number one cause of illness in travelers. Travelers’ diarrhea can be caused by viruses, bacteria, or parasites, which are found throughout the region and can contaminate food or water. Infections may cause diarrhea and vomiting (E. coli, Salmonella, cholera, and parasites), fever (typhoid fever and toxoplasmosis), or liver damage (hepatitis). Make sure your food and drinking water are safe. (See below.)
In other common man's terms, DONT DRINK OR EAT ANYTHING OR YOU WILL HAVE THE RUNS FOR DAYS AND HAVE TO SPEND YOUR ENTIRE VACATION SITTING ON THE JOHN!!! If there are pieces of animal feces floating in it, DONT DRINK IT!!!
Other problems include eating meat that has been sitting in the warm mexican sun may not be very good for you to eat, but that is just a theory, and "has never been proven true".
Hmm, time for a doughnut, and if you are interested, the site from which I got my information is as follows -
http://www.cdc.gov/travel/camerica.htm -