Here's my take on this meme.
Wars over land, oil, women, etc. are fought and won. The winners take the spoils and enjoy them.
If the war is over water, then their enjoyment can be spoiled by the losers, who have many and easy ways of destroying the quality of water.
If this should happen, then both sides lose, changing war from a zero-sum game into a negative sum game.
Addendum: This is basically an ancient form of mutual assured destruction.
...and this is the reason that we see so few water wars (despite the headlines) -- human culture has evolved away from them.*
Bottom Line: Water wars are possible, but they are rare because they are so hard to actually win.
* Of course, that doesn't mean we do not see cruelty over water management, as Amnesty recently pointed out with the Israelis' treatment of the Palestinians. (And, worse, Palestinian politicians' cynical exploitation of their "constituents" misery as a bargaining chip!)