I am going to Mexico for 16days in July with my girlfriend to visit her family. I will be in Monterrey (Mexico, NOT CA) and Cancun, my question is about the quality of the water, and if it's safe to drink. I found online that it is not recommended to drink the water or use it to brush your teeth etc However, my girlfriend just laughed when I mentioned that....

I don't want to look like a twat in front of her family treating water with a steripen or just use bottled water, but I would rather not fall violently ill ruining the trip either.
Anyone who's been and can give some first hand knowledge would be awesome!
