Shortages of clean drinking water or agricultural irrigation water are nothing new. Even the Romans built aqueducts. Like shortages of food or housing or space it is not the worldwide supply that is the problem, but the failure to get the needed asset to the people that need it and it is the greed of men that keeps it that way.
It is the modern water purification systems that have enabled us to have 7 billion people to start with. Over 100 years ago you could never have a city of 10 million people, they would all die of disease in their own sewers. The fact many people want to ignore is that the population of the world is a self adjusting number. If we get too many people, sickness, starvation, war, and other things will cut it down. When we make new discoveries and learn to provide more food, water and housing, like we have in this industrial age, then we allow that population to grow.
As far as innovation in desalination, it will come, when the need it great enough and someone can make money off of it. That is the plus side of greed.