First off, the earth's surface is made up of over 70% water. In short, there will never be a lack of water*. Now, yes, the issue here is drinkable/potable water. Well, that's where technology and human ingenuity comes in. Right now we have fully functional desalination plants all over the world. And more will be created in the future. So, unless you kill all the scientists right now who are researching better means of desalination (the process of removing salt and other substances to produce clean, drinking water), the future is bright when it comes to water. Another issue is transporting water from the coast to the inland areas. But then transport is transport--human simply have to create better ways to transport huge amounts of water "uphill" rather than "downhill" (i.e. like ancient Roman aqueducts). Pipelines etc. that pump/carry water inland are not impossible to build.
So my stand is NO: There will never be a war over water. Man will only remain warlike as long as he still feels the need for these three "G's"...Guns, Gold, and Glory. I personally believe that somehow, if mankind as a whole gives up these three things, the world will be a more peaceful place to live in.
*well yes, in about 25 billion years or so, the sun will undergo a slow growth to become a "red giant"...and thus while it is growing to this immense size, the oceans of the earth will start to boil away because of the excessive heat...but you don't get to live up to 25 billion years old right hehehe so that's not our worry.