Drinking water will be one of your substantial expenses in this climate.
This 950 ml bottle of water costs 5 baht if you want it cold, or 25 baht per 6 bottles if not from the refrigerator (in some shops it costs 20 bahts for 6 bottles, at least here in Chiang Mai). It is sold not in 7-11. Look for it in small shops.
Seven-elevens are actually not cheap. A bottle 650 ml is sold there for 10 baht, and 1.5 litters for 14 baht. A cheaper option in 7-11 is to buy a 6 L bottle for 38 baht.
In Tesco, you can buy a pack of twelve 600 ml bottles for 39 baht or six 1500 ml bottles for 56 baht.
Drinking tap water in Thailand is not a good idea.