Oil is constantly spilling into the ocean at small rates, and has been for eons. Without these bacteria it would build up forever, fortunately as long as the rate of introduction is not excessively high it gets metabolized.

> Or eating the plastic that the oceans are filling up with?

Not yes, so far as anyone knows. But they, without a doubt, eventually will. Plastic has lots of energy in it, and is made of the exact same atoms life is made from.

I suspect that plastic degrading microbes would result in more CO2 being released into the atmosphere. Unless there is some sort of carbon sink in the process, I believe carbon in plastic form is less damaging than CO2.