Here's an article describing that in somewhat greater detail:
The idea is to heat them very hot in the absence of air. I remember doing a student lab experiment in the destructive distillation of wood.
The idea is to get back petroleum distillates from heating the plastic. So your ship could in theory fuel itself on the distillation products. Now whether it could do so cleanly is another matter. You have to ask whether you can build a ship's engine that could burn the products of destructive distillation (and for that matter, keep the furnaces going) without causing worse environmental damage.
Frankly, I'm skeptical of that. It has the look and feel of "perpetual motion" about it: trying to get more energy out than you put in. You'd have to build a nuclear-powered ship equipped with a furnace (to use as the "destructive still") and huge tanks to store the distillates. You could then off-load those when you make port. But the idea would not be to get a source of energy. The idea would be to clean up the ocean.