Well, I'm not so pessimistic about this. Although water is a scarce resource, by using it efficiently we'll have unlimited water forever.
We've been treating water as an infinite resource but this will change once we realize of it's a finite resource.
Two reasons for my optimism:
1. Population. Contrary to popular belief, population is already shrinking in most places. Those families of 10 children are a thing of the past even in Africa or Asia. The future will be less crowded but with an ageing population and this will bring other issues.
2. Technology. Once more, technology will come to the rescue. There are countless ways to produce, save and use water efficiently. These methods are getting cheaper with technological advances and scalability.
Humans are extremely resilient, we have survived for millennia in the Sahara dessert, tomatoes are grown in the middle east and desalination with solar energy is becoming a real possibility.
Technology is the cause and the solution to all our problems.