If I'm flying in the back of the plane, I usually bring two empty two-liter soda bottles in my carry-on, to fill at a water fountain, and have never had a problem, since the whole liquids ban started.

If you find yourself having to go through security again, simply empty it out (restroom/water fountain/outside, maybe) before heading into the new security line. If the empty bottles seem bulky, I'm sure there are some collapsible camping-oriented water containers available, which would take up less space in your carry-on when empty.

I've even considered taking my camelback through dry, filling it at a fountain, and just wearing it...