What makes you think this doesn't happen already. Arizona called in the National Guard on California during construction of tunnels out of Lake Mead. Mexico has threatened to take the US to international court over the salinity of the Colorado River. Dynamite was set off numerous times on the aqueducts that bring water to LA. Hostages have been taken and killed over the construction of dams in South America. Fresh water already is the scarcest resource we have in the American West. Far scarcer than oil by a long shot.

Water will never become a valuable commodity for the foreseeable future (on a global scale). Most of the Earth's surface is covered by water, and we do very little that actually destroys water. What may become valuable is drinkable water, which is easily obtainable from sea-water (for the right price), an local access to water.

The first of these problems has an easy free market solution, and sets a hard cap on the value of drinkable water (as de-salination plants can scale linearly to meet our needs). The second of these is more difficult, but still has a worst case scenerio of needing to move people to water rich areas, and re-build infastructure to support the new population distribution, as well as dealing with a decrease in the habitable land area.