97.5% OF WATER IS SALTY OR POLLUTED, ONLY 2.5% OF WATER IS CLEAN, FRESH AND DRINKABLE
A lot of oil, especially coming from the middle east, when they spill oil into the ocean, it ruins the eco-system and a lot of organisms die when oil is swallowed.
Metals like mercury, lead, iron, magnesium, and cadmium have an unfortunate tendency of being ended up in the oceans, where it sooner intersects food chains and cause metal poisoning.
Oil spills are also very common around the world. When the oil spills, the chemicals because acidic due to the reaction with sea water, therefore spreading the dangerous chemical around the ocean and kills a bunch of sea animals.