Fresh water isnt a "public good", technically or economically speaking. A "public good" is a commodity that is neither rival or excludable. This means that the quanity or quanity is not deminished by people using it, and that you cant prevent anyone from using it. A lighthouse or public radio are examples of public goods.

Fresh water, economically speaking, is a classic private good. It gets consumed as someone uses it, and can people can be easily restricted from acess if they dont pay.

https://en.wikipedia.org/wiki/Public_good_(economics)#Defini...