Why is water pollution dangerous?

Water pollution is dangerous for both nature and humans. If all water sources are polluted, then fish and other organisms will die in it. And people will have nothing to drink, because there is so little fresh water on Earth. It is also dangerous to swim in dirty water, because it can get infected with various infections, such as lichen or E. coli. Therefore, water must be protected, because without it there will be no people, no animals, no plants.