Yes they tell you a lot of simplified things in school, I went to school in the 90s too so I remember the lessons on the water cycle and water table depletion and stuff... back then people were worried about water wars between Isreal and Jordan for example but they now work together on a desalination project which is refilling the Sea of Galilee. Things have changed since we were in school.

Freshwater is just clean water, we have loads of it fall from the sky and we can take dirty water and clean it even when it's salty. As long as we have dirty water we can make clean water and the planet is like 70% dirty water so honestly we're more likely to run out of pretty much anything else. P