In my opinion,Thailand is a relatively clean country.Anyone who thinks otherwise-go and visit India!I always brush my teeth with the tap water in Thailand-no problem.I only drink the bottled water tho'.
I think to drink their tapwater is foolhardy.
I drink the iced water from the noodle stalls-no problem as they get their water from those purifying machines you see everywhere in Pattaya /bangkok.But given a choice ,I would drink the bottled water.
I had the shits a few times,but I never put it down to the water,I think its been salad that was washed in ordinary tap water that gave it to me.