The water cycle
Salts do not evaporate. So, rain water, even if that comes from the (salty) oceans, does not take salt with it; it is 'sweet' water and therefore suitable as drinking water for men, animal, plant.
But during the process of flowing back, via rivers, to the sea, that water absorbs small amounts af different salts and that's how the sea collects every time more salt.
Lots of rain water does not enter the rivers at all, but remains in the bottom of the earth. There people find most drinking water today.
This is normal; it goes that way for millions of years.
But something becomes disturbingly when rivers do not longer transport natural salts to the sea, but also polluting salts, mostly caused by fertilizers and insecticides. Nitrates, ammonium salts, etc are very well soluble in water, and difficult to filter out.
If that comes into the rivers, it is transported to the oceans.
Even worse when it enters the sweeet water stock for drinking water.
Apart from artificial manure, there are many other sources of water pollurion: industrial waist, wrong substances that through the air enters the water, like Sulphur dioxyde, etc.
To clean all this, we have different methods, for example the precipitation, filtration, treatment with Chlorine, etc.