I'm traveling to Puerta Vallarta, Mexico. Everyone who I have talked to has said the same thing.
"Be safe!"
 "Don't travel alone!"
 "Don't drink the water."
I'm clearing up this rumor. Not all of the water is unfit for consumption. Some parts of Mexico have been found to have drinking water that makes tourists sick. That much is true. Some towns have more stringent filtration systems. But it's not because the Mexican people are dirty, or because they are less advanced. The tap water sometimes contains bacteria that our bodies are not use to, and filtered water is available if you find yourself in an area where the water is not safe. Yes, they are able to filter the water from taps or springs so it is safe for consumption. How else would the Mexican people survive? Cook? Shower? Brush their teeth? According to Mexperience, water in Mexico can be delivered one of three ways: via main-feed, via a communal feed sourced from local water springs, or a combination of rain collection and local water delivery by truck. Some property owners even install on-site filtration systems. As for the hotels and resorts, like where I am staying, most of them have a filtration system, such as desalination. For those of you who don't know, desalination is the process of taking the water from the ocean and filtering the salt out to make it safe for drinking.
So you see, it all depends where you go and what kind of experience you'll have. If you stay with a local family in the country, you might consider researching the filtration system, but be respectful of the people in the community, and be careful how you approach the situation. If you stay at a resort, the water is likely safe.If for some reason it is not safe, the hotel will provide you with filtered water. They won't let you get sick. I have friends who have traveled to Mexico in the last couple years, and they all said there was nothing to worry about.
Go ahead. Brush your teeth, and order that margarita on the rocks. Life is too short to not enjoy your vacation because of a few bad experiences.