It's not an existential problem so much as a political problem. We can literally always boil water. The planet is 70% water and it's not going anywhere or getting converted into other things.
The level of ground water is going down continuously and sea water is not a great source of drinking water yet. Yes I wish there was a great technology to desalinate sea water efficiently and cheaply.
Boiling requires energy and doesn't remove the salt. Cleaning polluted water can be even more energy intensive. So it is getting converted from fresh water into polluted water, all the time.
Boiling sea water for fresh water is a solved problem, its just that nobody wants to do it.