I've yet to step foot in an airport that didn't have water fountains, and won't let you take an empty reusable bottle through security. There's no need to buy bottled water.

I take empty bottles always, but most airports have no fountains and only hot water in the bathrooms.

I don't remember any fountains in the Warsaw airport.

The exception that proves the rule (else it wouldn't be a rule, but a universal law that goes without saying)...

http://comicfury.com/forum/viewthread.php?id=26895

Heathrow, too, the last time I was there—though it was a long time ago. (I actually asked.)