Question

how does water pollution affect the environment

Answer 1

Addition of harmful and undesirable substances to water as a result of human activities is called water pollution. 
When humans dump their garbage in a water body their water gets polluted, also the use of fertilisers causes water pollution as the chemicals flow with water to the water body. Dirty water also liberates foul smelling gases.
Dirty water gives rise to mosquitoes of malaria, dengue, etc., which causes illness in humans. The fishes living in that water get poisoned and when humans consume it, they also get poisoned.