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.