Can you drink the tap water?

Japan

Yes, anywhere in the country — including from the drinking fountains in stations and parks.

China

No. Boil it or drink bottled — the kettle in every hotel room is there for exactly this reason.

Thailand

The Bangkok utility certifies its supply as safe and drinkable. What that certification cannot cover is the building you are standing in — or anywhere outside its three provinces.

Vietnam

No — the instruction from the tourism board is to stick to bottled water. Read the rest of that sentence carefully though: the caution about ice is written for rural areas, not for the whole country.

South Korea

Yes — it is safe and heavily tested. Almost nobody drinks it straight anyway, out of habit rather than doubt.

Taiwan

In Taipei, yes — the city's water department certifies it directly drinkable. That certification is Taipei's own, and it does not travel with you.