I've never read a scientific study regarding this, but I would imagine that the water, while not filtering out THC, might take some junk out, but the unglamorous truth is that no matter how much water you run it through, you're going to pay for it sooner or later with either diminished lung capacity or even, god forbid, lung cancer of the big E.

Just a guess: bongwater probably sucks 25% of the by-products of combustion out. Either way, you're still setting a plant on fire and sucking the smoke into your body!