This basic human right that a lot of us have the privilege of acquiring however much we want whenever we want, is severely undervalued in the global economy. Water is crucial to existing and flourishing on earth, both for our human communities and our natural environment and you can't really put a price on something that vital. That being said, putting a price tag on it might be the only way we can ensure we use it responsibly. I envision a future where everyone has access to water in an equitable and necessary way, major emphasis on those last two words. Water crises around the world have shown us time and time again that this fundamental resource is actually finite and we don't have an abundance of it (contrary to what our water usage habits exhibit). The way the West and industrial corporations use exorbitant amounts of water yet pay little to none to use it, and leave very little of the resource for the rest of the world just furthers the inequities individuals in lower socio-economic demographics already face.
In order to combat this unequal sharing of water, I believe some sort of compensation system should be in place for different sectors. For example, at the individual level I think water should be priced based on income and incentivized so that the first 100-200L are free and then beyond that there are different stages of pricing. At a larger scale, industries that use ridiculous amounts of water in their processing and manufacturing should have to pay some sort of "water tax". Hopefully, this helps regulates water usage and eventually decrease it to levels that are replenishable and we can push back our Day Zero. Implementing various water purification technologies is also a start, funding should be prioritized for communities where water is scarce so they can install water purification systems of their own. Through all these different avenues, an invaluable yet slowly depleting source will hopefully be seen as liquid gold and not taken for granted.
I think your idea of water tax can be quite beneficial in terms of reducing the overall water wastage, especially the heavier empha