Water pollution occurs mostly, when people overload the water environment such as streams, lakes, underground water, bays or seas with wastes or substances harmful to living beings.
Water is necessary for life. All organisms contain it, some drink it and some live in it. Plants and animals require water that is moderately pure, and they cannot survive, if water contains toxic chemicals or harmful microorganisms. Water pollution kills large quantity of fish, birds, and other animals, in some cases killing everything in an affected area.
Pollution makes streams, lakes, and coastal waters unpleasant to swim in or to have a rest. Fish and shellfish harvested from polluted waters may be unsafe to eat. People who polluted water can become ill, if they drink polluted water for a long time, it may develop cancer or hurt their future children.
The major water pollutants are chemical, biological, and physical materials that lessen the water quality.

I agree that humans pollute the water with trash and chemical waste, but in addition our carbon emissions also contribute to worse water quality by acidifying the water according to the National Resources Defensive Council (https://www.nrdc.org/issues/reduce-ocean-acidification). Ocean acidification occurs by our excessive carbon in the atmosphere and the increased acid in the oceans can disallow organisms to draw molecules from the ocean to form its shell, so the organisms energy would focus more on shell formation rather than finding food or a mate, which would end in their life. If the water acidity is high enough, then organism shells disintegrate. This is another reason why we should be cleaner in our carbon footprint.