What will happen if water is polluted?

It depends on what the pollutant is. Some chemicals build up in the body over time (like heavy metals and chlorinated hydrocarbons). These cause problems like liver failure.

Acidic and alkaline pollutants can damage the tissues in the mouth and digestive tract, killing cells and causing trauma.

Toxins like arsenic and cyanide can cause rapid death by killing cells, Cyanide, for instance, bonds to red blood cells, making them useless for transporting oxygen and killing the animal by asphyxiation.

Most minor pollutants cause minor effects like dihorrea and vomiting, but these can also kill as they cause severe dehydration.

If the pollutant is biological, like bacteria, these cause illnesses like gastroenteritis and other infections.