The last time we were in Mexico we did a tour that went to a Mayan village, the ruins at Coba and a cenote. The guide we had was excellent! What he explained was that the water in Mexico isn't bad. It's what's in it that our system isn't used to. Mexican people are, and he said it's likely that if they were to come to Canada and drink the water it would upset them in the beginning too, largely because of the chemicals and filtration that our water goes through.
The thing is, you do have to be careful with the water, and that includes brushing teeth, which a lot of people forget about most resorts will provide bottled water anyway. Fruits and vegetables are washed in water so occasionally you might get an upset. Yes - a lot of the resorts are now doing their own systems, and a few of them are joining together with the cost and sharing systems. it gets harder when you get to the eco protected areas. Our guide told us that it's something that Mexico takes very seriously because they are so dependent on tourism. Having said that we've been to Mexico 5 times and never, ever had a problem at all. We went to Cuba once and were both as sick as dogs. Go figure.
I would advise if you have new travelers in your group that you recommend immodium, or its equivalent in case anyone does get an upset. There is also a vaccine that you can take - Dukoral - now available over the pharmacy counter here in Canada that will help prevent the travelers' upsets but you still need to use your noggin and watch what you do. Some people are definitely much more sensitive than others.