We always ask local people about water quality - there are lots of places in Mexico where drinking the water is OK - always ask - but don't ask Mexicans - ask gringos and fellow RVers who do not have the same immunities the Mexicans have. If they say the water is OK we have been drinking it with no problem and give it to our dog too. But if we are drinking bottled water the dog is too.
We bring large bags of dog food and cases of canned dog food every time we go to Mexico - enough for our planned stay and then some. We have never been questioned or had a problem. If a pet requires any special food I would not expect to be able to find it in Mexico - very much depends on where you go. Though we carry all her health certificates etc. we have never been asked for them when crossing the border in either direction.
Our dog is always on a leash - it is our responsibility to protect her and we do that by leashing her - so she never has a chance to drink from puddles or get involved with ticks or fleas - so far. Of course there have been occasions when she has been off leash and she learned to eat donkey poop so we put her back on the leash. We also always pick up her poop even though Mexicans seem to think we are crazy for doing this. I don't want to step in your dogs poop any more than you want to step in my dogs poop.
