There have long been rumors about enormous dumps of plastic in the vastness of the Pacific Ocean, where all the world's plastic bags and styrofoam cups get whirled together by currents into islands of debris. Alas, expeditions launched to find these floating trash dumps were generally disappointing, leading to all sorts of strange theories about plastic sinking to the bottom or getting pulverized into tiny pieces.
Turns out that most of the plastic that goes into the Ocean dissolves. Warm salt water can turn most plastics back into hydrocarbon molecules in a matter of months. While this discovery frees us from the nightmare of the seas getting covered by continent-sized mats of plastic trash, it also means that plastic washing into the sea is just like piplines disgorging chemical waste.
And while I'm on the subject, it's worth pointing out that most of the plastic in the ocean doesn't come from cruise ships or nefarious waste dumpers, it washes in from the land. Litter = chemical pollution of the oceans.