I think it can depend upon where in Mexico you are going and what the prevalent problems are there. In the Baja, we have to be very careful of erlichia which is transmitted by ticks. So, even more than the water, we keep our dog on Revolution or some flea and tick treatment.
We also watch him on the beach because some fish are poison like the puffer fish. Eating those will land you quickly in the vet's office.
So yes - dogs do drink from the toilet, lick their own wounds, etc. and are often fine. But in the US they are not exposed to the same kinds of things they will be exposed to south of the border. We prefer to be safe rather than sorry.
I suspect everyone will have a different perspective and different experiences. We too lost a dog last year in the Baja - not from the water but from a disease transmitted by ticks (erlichia) and we didn't understand how easily this is transmitted and how difficult it is to treat in some dogs. So I mention it here in case you too may be unaware of it.
