How does plastic end up in the watershed?
You may be wondering how a piece of plastic litter can get all the way out to the middle of the ocean. Think of it like this: you drop a plastic cap on the street and later that day there is thunderstorm and the rain washes that plastic cap down the street to a nearby storm drain. That storm drain empties out to the closest body of water from where you live. That body of water travels from river to river, until it makes its way to the open ocean. That means anything in your storm drain, including the plastic cap, goes into the ocean.
It's important to understand that no contribution to stop usage of plastic is too small. You may think that it doesn't matter whether there is one little plastic cap in the entire ocean, but if everyone has that kind of mentality, then everyone will litter and soon enough the entire ocean will be full of plastic and trash.