There really is no place to dump water that you've forgotten in your bag if you want to keep the bottle. I suppose you can dump the water in the "restricted items" bin they have at the beginning of the security line (looks like a garbage can), but if you pass it and forget you have to drink it all right there. That's what I've done. I figure my chugging only holds up the line for a couple of minutes.

Honestly, isn't there some kind of litmus test the TSA can use to tell the difference between liquid explosives and the building block of life (water)??