I just looked it up. Estimated total quantity of plastic produced to date since plastic production started is about 10 billion tonnes. Estimated total weight of seawater in the world is around 1.5 billion billion tonnes. So even if all that plastic was in the seas, there would be 150 million times as much water as plastic. In reality it's only a problem when the amount of plastic particles in a particular location is very much higher than the global average.