The most polluted oceans are the North Pacific Ocean and the Indian Ocean. The North Pacific Ocean has more that 2 trillion plastics that are floating about in it which is making it toxic, like a garbage can. The Indian Ocean is a close contender and is also filled with garbage. Those two are the top polluted rivers as far as non-biodegradable items being in them. The Baltic Sea is also very polluted. It is high in pollutants from ships and from factory run offs and that is especially bad because it is a prime fishing spot like for fish, crab, and other seafood. The Gulf of Mexico is also very polluted. Oil spills from tankers pumping and looking for oil is a big problem in that area.
The North Atlantic is up there in pollution as well and the Arabian Sea doesn't fare too well either. The level of oxygen is diminishing in the oceans and seas that are heavily polluted which, of course, makes the seafood within the waters lessen and also makes them not healthy to eat. We are killing our oceans and that is the bottomline. The sad thing is it can be stopped by most people don't see the devestation so they don't really catch the magnitude of the problem.
The most polluted ocean on Earth is the North Pacific. This throws the blame on North America and other developed countries whose coastline is on that ocean. The plastic waste is 900 times greater than that in the Indian Ocean and a huge mass of such waste formed a kind of plateau between the US and Japan.
The North Pacific Ocean holds an estimated two trillion pieces of plastic representing a third of the total plastic found in the ocean, and there is more beyond plastic.