It seem impossible on a planet that is 70% water, but affordable, potable water is getting more and more rare, even in the United States.
See this documentary Flow: Love of Water
to get a survey of the many aspects to the problem.http://www.flowthefilm.com/aboutwater
See the April issue 2010 of National Geographic - all of it devoted to water use around the globe.
70% of the earth's surface is water, but 97.5% of that is salty. The UN predicts that within 20 years over 5 billion people will face water shortages.
Desalination might be an obvious answer, but that requires energy. And we're running out of that, as well.
We need to change our habits.
The NatGeo magazine site has some of the April issue online:http://ngm.nationalgeographic.com/2010/04/table-of-contents
The main problem is lack of clean water in third-world countries, but even where water is available in the developed countries the cost of supplying it is rising. Even in the UK, where it is always raining! For many years we have talked about fuel poverty. Now, with the recession, we are also beginning to see water poverty. The Government has defined it as "when water bills make up more than 3% of a household's spending" and is considering seting up a £500m support fund.