That's just not very likely to happen from a basic chemistry point of view.

Plastic tends to be quite stable--that's the whole point. However, that stability means that there is almost nothing to be gained by breaking plastic down--certainly not energy. And, in the ocean, practically everything plastic could provide when broken down is far more abundant and easier to access by other means.

Consequently, there is no good reason to evolve the ability to consume plastic.

Given that amount of plastic waste that humans put into the ocean is already on the scale comparable with a mass of a lot of fish species, an organism that can consume it can have a nice evolutionary advantage.

> That's just not very likely to happen from a basic chemistry point of view.

Actually, it has already happened (DOI 10.1126/science.aad6359, http://science.sciencemag.org/content/351/6278/1196). However, do note that there are many types of plastic (https://en.wikipedia.org/wiki/Plastic#Types) - the linked article is about degradation of PET specifically.

Why?

Because the energy expended to break down the plastic means that it is outcompeted by the bacteria who get their materials by other means unless plastic is the only thing available.

In the ocean, there is always lots of other things available.

Perhaps this will turn out like the evolution of lignin-modifying enzymes, but those took a LONG time to come about even while lignin is far less stable than most plastics.