In most of the airports (if not all) that I have been lately, there are water refilling stations, usually close to the restrooms or after you pass through the security checkpoint.
I usually pass security with my plastic bottle empty, and then refill it with those machines.
Nevermind that, I once tried to board a flight in Dubai with a completely empty water bottle. Just before I boarded the plane, security stopped me and rummaged through my bag for a minute or so as though they were looking for something then just threw my empty bottle out, saying it was not allowed. I was thirsty for the whole flight because of that. They did, however, manage to foil my plan of bringing down an A380 with an empty 200ml bottle.
This is fine for domestic travel, but not nessesarily for travel to the US. Many of these flights have additional security at the gate just prior to boarding. US is way too paranoid
