Drinking and Water Safety in Mexico
It is an age old question for those traveling to Mexico or any country that does not have 100% reliable water supply. This is because of the notorious Montezumas revenge. So can you drink the water? 
Of the course, the answer is.... it depends.
Resorts
On resorts, the water is generally 100% safe. The Vidanta Resorts have their own water purifcation system. The water might taste a bit chemical, but it is perfectly safe for brushing teeth and drinking. We prefer to head to the local supermarket for drinking water. You can buy 1 and 2 gallon jugs for a buck or two. You can get large jugs in the La Plaza in Neuvo Vallarta or in Riviera Maya. Or you can use the water provided by the resort in bottles for a price that works out to about $20 / gallon. Just like nice hotels in the U.S. and Canada.
Restaurants
In the main tourist areas , almost all restaurants use purified water for ice and the water is safe. If you are one of those people with who will worry and not enjoy yourself, simply ask for bottled water and don't get drinks with ice. In mainstream areas, I never do that and I never have a problem. Recently we did a walking food tour in Puerto Vallarta and learned that all the street vendors on the Malecon use purified water and are licensed and inspected by the City. However, it is true that the more "authentic" the area the less likely the water is to be safe. If you stay in an apartment in old town Puerto Vallarta or one of the small villages in the Riviera Maya, better to be extremely cautious. Don't brush your teeth with the water, don't drink it, don't swallow it in the shower. Wash your own hands vigorously with soap and dry thoroughly. You may want to take hand sanitizer as well.
Local Hotels
The rule is if they tell you not to flush the toilet paper, the water is not safe to drink, brush teeth etc. Use bottled water to brush your teeth. You may want to put something on the faucet to remind yourself. Habits die hard. If cooking yourself, you should soak dishes after washing in a mixture of 2 teaspoons of clorox bleach/gallon of water. you can by products like Bacdyn to soak fruits and vegetables. Follow the directions carefully
Two Final Points
1. Travelers diarrhea is common no matter where you travel. Just changing food type, a new climate, sunburn and time changes can all cause disruption of the normal digestive process. So , don't get too paranoid. You definitely should not take prophylactic antibiotics, but you could see your doctor for a prescription of a drug similar to ofloxacin and also Immodium (lopereamide). But no need to obsess about one or two loose stools. It is severe illness and a fever that should trigger you to do antibiotic and antidiarrheal treatment. One of my friends chews peptobismol tablets 4 times a day. I wouldn't do it , but if you are going to worry too much, go for it.
2. Don't drink a bunch of tequila shots and then try to blame the water. Also,choose a better quality tequila for a better outcome.
So go forth with confidence using this is a summary of water safety in Mexico and when to drink and not drink the water.