We currently dumps millions of tonnes of plastic into the oceans, every year. This huge spread of waste is taken by the currents, and ends up reaching across the globe, contaminating every part iin time. Plastic's damage to the marine ecosystem is most obvious with larger creatures, such as turtles, dolphins, and birds that die as a result of the waste. But the biggest danger is in fact the chemicals that leech out of the plastic over time, and kill of the plankton that is vital for everrything else to survive, directly or indirectly.
And this plastic waste is not going anywhere - it takes centuries to even begin to break down, wrecking the marine ecosystem constatly, and by extension, the ecosystem of the entire planet.