The smaller bits of plastic absorb all kinds of toxins in the ocean (that in itself ought to be of some concern), and marine life eat them and get poisoned. But they might end up floating anyway. The majority of the plastic seems to be in the form of tiny pellets that apparently do quite a lot of harm that way.

A better regulation would be to require them to not throw plastic overboard at all. I don't know how big that garbage patch has to get before the people who make those regulations figure this out, though. People are really stupid. :-\