Don't worry, humanity ain't gona really do anything about the supply of fresh water going down. Why? Because our economic model sais that the lower the supply of a material (water in this case) the higher the price of that material and thus more profit. Our economic system is designed, rather well, to maximize profit (consumtion).

What's going to happen I think is that water price will go sky high and then people will start investing in desalination plants and waste water treatment units because the water will business will be very profitable. There's no technical reason why we don't this now so as to avoid even greater scarcity of fresh water, but we won't because it's not that profitable at the moment.

I mean the actual statement I should have used is "why we haven't already invested in desalination plants and waste water treatment units", because we've known that fresh water will become more and more scarce for quite a while. Here's a link to a report from 1998 talking about the scarcity of water at that time, and predicting greater levels of scarcity for the future.
https://www.k4health.org/toolkits/info-publications/solutions-water-sho…

Love & Peace
Rayko