Thais mostly drink bottled water, which is clean. They aren't drinking river water or something. Most of Thailand lacks drinkable tap water. Food hygiene standards are more lax than in the West though.
Thanks for the inside info. But do they brush their teeth with bottled water? Do they wash their dishes, tomatoes with bottled water or with "dirty" tap water?
Sarcasm aside, I'm sure it varies from place to place, but in places I've been to in Thailand (and I live half there, half EU), tap water is fine for washing/brushing and if anything, too chlorinated (and thus not too drinkable). I still drink bottled water (or filtered water; many local friends have filters installed to their tap).