In the seminar on the Pacific Gyre I saw, the guts of the fish they caught, had tiny little flecks of plastic or balls of plastic in them. I'm sure you wouldn't want to find that in you sushi roll.
The plastic as it breakdown, and gets eroded by the ocean forms little plastic balls. Here's a picture of the sand at Kamilo beach in Hawaii. All those little round things are plastic.
And if you need more convincing see this BBC video