Quite an interesting question and one that required both research and introspection. I finally had to rely on what I have experienced in travelling to 6 of the earth's continents.
The world is losing potable water at a rate that is unprecedented. Saharan and sub-saharan Africa water supplies are contaminated with bilharzia, schistosomiasis (sic), giardia, and other water-born parasites and diseases. Russia has seen the Caspian Sea and other land-locked lakes decline rapidly over the past 25 years due to evaporation from a warmer climate and drainage to irrigate arid lands. Populations are expanding into areas where water is scarce and aquifers will be drained in less than 100 years at current consumption rates. Even my favorite Sierra streams are polluted with giardia and require treatment before you can drink the water. China, India, Asia, and South America are also faced with the same problems of water-born diseases and decreasing supplies.
In the 19th century we saw battles fought over water rights in New Mexico. What will happen in the next century when nations are faced with hydro projects in a neighboring country decrease the flow of water to another country. This is already happening in Turkey where a massive hydro project threatens Iraq's water supply. Will this be Saddam's next war?
Scientists still do not understand what the impact of desalinization of sea water will have on the balance of the global ecosystem. The salt removed has to go somewhere, usually back into the water. Will increased salinity impact polar ice caps or the temperature of the earth's oceans? We have already seen an increase in the salinity of water in Russia, the Middle East, and even in areas in the USA.
In terms of consumption, we waste a tremendous amount of water as individuals. More water goes down the drain or the gutter as we continue to plant water-intensive lawns or keep the tap running while we brush our teeth. Recent draughts in California have highlighted how much water can be saved on a daily basis. On the converse, in many third-world countries more time is spent each day bringing the necessary water to the home as wells dry up. You can't be productive if you're always fetching water.
The next global war will be fought over water, not imperialism or political ideology. Man can live without a lot of things, but not water. Mainstream media in all countries have been absent in their reporting as well as conservation groups like the Sierra Club. What will the world do when there's not enough rain to go around?.