This water is polluted with litter and hidden bacteria
Without doing detailed scientific tests, it is difficult to know how polluted an area of the sea is. When pollution is at its worst, dead sea creatures may be washed up onto the shore. There are many different materials that pollute the sea.
Some materials that escape into the sea will decay. The decaying process is done by bacteria that may also release harmful poisons, or the bacteria when they get into the bodies of sea animals through their gills or mouths make them sick.
Some pollutants are chemicals that affect the health of sea creatures. An affected animal may become sick and become easy prey. The chemical may then affect the predator. Sometimes predators build up large amounts of polluting chemicals in their bodies.
Bacteria can use most of the oxygen, when they decompose grass, leaves, dog doo and other organic material.