By "safe" you mean non-deadly? And you mean tap-water?

Probably, at least in Bangkok - where the water quality has greatly improved over the last 15 years - but most Thai people I know get bottled water delivered, as do I, and use that for drinking, ice, and cooking. Tap water is fine for laundry, general household cleaning, showering.

Obviously, in a life/death situation I'd drink the water in Bangkok, 50-50 in Pattaya, maybe 0:100 after a flash storm?