What part would those beasts who live in the water play in working to clean it?
You dove into the poisoned waters of the Sea, bearing small robots with lengthy tethers for power and sacks that could balloon immensely; each one would draw water through, filtering plastic, oily residue, and the rotting remnants of algae that sucked the oxygen out of the water. You merely needed to return to them once a week or so, hauling each hefty sack back to the surface for those dwelling on the land to collect and dispose of, by recycling, by arc furnace, or by compacting and sealing it away in a deep mine to never again see daylight.