I want to go to Japan and South Korea. I don’t know whether the custom there is to drink boiled water or water.

In Japan, when you go out to eat, you are given water with ice cubes all year round. It was a bit unfamiliar when you first went there. I had a stomachache on the first day, but the medicine for stomachache in the convenience store was quite effective. I'm feeling better now, and I don't have any more pain. Or you can buy a thermos cup. There are hot water kettles in hotels or B&Bs, so you can boil water yourself and bring it with you.