Although tap water in Thailand always looks fine to me, it is not recommended for drinking.
But fresh drinking water is cheap and easily available, so everyone - even locals - buy it. Some restaurants even get their ice cubes delivered. Sometimes several times a day.
This is how we get our drinking water.
From a little corner store a few doors down we purchased a big 20 Liter water bottle (as seen in the picture). Now, when it gets empty, we put it out on the side of the street in front of our house.
The water delivery trucks go down our street several times a day. When there is an empty bottle put out, they stop and exchange it for a new and full bottle of drinking water for just 12 Baht, which is less than $0.50 USD.
We have even gone as far as putting the 12 Baht on top of the bottle - just in case we don't hear them coming/stopping and have never had any issues with that yet (as in someone stealing the bottle or the money :)
Pretty neat system, eh?