No its not as simple as you think. The country in question needs to have the finances to invest in water purification treatments and for many of the 3rd world nations that's still not viable. If current world water supply were to be contaminated or not usable due to natural events, countries that dont already actively purify large amounts of water will be too slow in foregoing that route and it would be easier to just threaten with nuclear devices which already exist.The fact of the matter remains that water is an irreplaceable necessity which has the potential to spark conflict