My belief is: at some point it will become clean water.
I remember the day when clean pure artesian water flowed freely. It still must exist somewhere in the world. Tasting it once more before passing on is on my bucket list. It's hard for young people to imagine clean air and clean water...both at the same time. It did exist at one time, now just a fond memory for many of us.
Clean water is becoming scarce. Fisheries are dying because the increase in ocean temperature, and the changing levels of salt.
However, the industrial countries (which are causing the problem) have the technology to counter it. So far. So they can get clean water, and create "fish farms", to continue to meet their needs.
And the non-industrial countries don't have a military capable of challenging the other nations. So we won't see a global war over resources. What is more likely, is an increase of state-sponsored terrorism as the "minor" nations attempt to "put the pain" on the "major" nations, to encourage them to change their ways. EG: attacks against water-treatment facilities, to deny their ability to get clean water.
To expand on your point, the world’s ancient aquifers, the freshwater reserves collected deep in the ground over millions of years, are depleting faster than rains can replenish them. Many farms are watered from wells tapped into these aquifers.
Walt O'Reagun 