We use VG/PG because it vaporizes at a lower temperature. Water vaporizes at, what, 212F? Boiling. So you don't want to inhale that anyway, you'd singe your lungs.
I don't know if we should be so quick to diss the possibility. There could very well be a reason that it's not possible but it's not vaporizing temperature. VG and PG vaporize at much higher temperatures, 554F and 370F respectively. Water is only 212F. (See boiling temps: Glycerol - Wikipedia, the free encyclopedia and Propylene glycol - Wikipedia, the free encyclopedia )
I'm not so sure that would work. As I understand it, the pg is in there not so much to provide the clouds of vapor (vg is much better at that), but to be a carrier for the nicotine and flavor. The same reason they use it in some medical inhalers. I don't think water would do that job so well. Also, nic bases are nicotine mixed with pg, vg, or some combo. It might be tough finding a base that didn't have either of them.
And as far as being carriers for nicotine and flavor: Nicotine is water soluble and so are many of the flavors we use. Water is the universal solvent after all.
If anyone's tried it, water is just so much less viscous than VG or PG, so just floods and drowns the atty because the wicks are designed for thick gooey stuff. Something that limits the flow could work -- like a much tighter/compressed wick. A mod could be possible -- I'm just sayin'.