Omisan said it right. Plus I'd say be aware of salads or raw vegetables in non-touristic restaurants as they may have been washed in water. Usually touristic restaurants use boiled water for our fragile stomach.
If you go eat in a private home, don't be shy to ask for boiled water.
There are kits you can buy for cleansing water, but I've never tried them. I had one with me in Asia, in case I'd go to remote places for a while and couldn't buy bottled water, but it stayed in my backpack the whole time.
In a private home, I once was given a bowl of noodle soup that I had assumed previously boiled but
nop, it was only hot water. 90 minutes later, I was lying in bed. Stomach in pain. For 24 hours.