In the 7-11s and Family Marts, plain bottled water is from 7 to 10 Baht. In the restaurants, they usually sell bottled mineral water for 40 or 60 Baht, about what you will also see in the small bars. In the stores, mineral water is around 20 Baht.
I got in the habit of ordering a bottle of Soda water at the end of the evening instead of a final beer. Helped me feel better the next morning. Soda water is typically sold for the same price as the cheep beers in the bars, or 80 to 100 Baht, maybe 120 in the go-gos.