How can japanese drink so little and still stay hydrated?

This baffles me every summer. My wife, my friends, my co-workers… They all seem to be living on one cup of coffee and a bottle of tea everyday.

When walking outside in the heat, I rarely see any japanese person whip out a bottle of water, not even next to the venting machines.

I don't understand how their bodies are able to function like this. I have raised concern to my wife many times, who claim that one or two bottles of green tea is all she needs while washing down two beers after work, which should dehydrate her even more.

Surely japanese bodies need the same amount of water like everyone else? Or is this another one of those nihonjinron cases, like the longer intestines? lol