Hey, what about if we equip a hive of small autonomous boats equipped with "sorbents"? They can have chemical detectors and share information from each other wirelessly, combined with aerial and satellite images. This could possibly enchance the cleaning efficiency. The "boats" don't have to be fabricated with expensive designs.. they just need to float and propell across the water. In fact, we can add solar panels so they can remain in sea longer (and extend mission range). A single "command" boat, operated by humans can be used to launch these small boats and observe their operation.
Some info of sorbents here:
Sorbents | Comprehensive Procurement Guidelines (CPG) | Wastes | US EPA
I skimmed through a waste water book. Ya know the pipes that go into the sea from the beach? It consist of processed waste water, but other wastes still get mixed into it. They are then diffused far away into the sea. Unfortunately.. a "plume" of waste can occur just below the water level.. so we might not actually see the extent of how polluted the sea really is. Anywayz, the book got into some detailed diffusion analysis.. I proceeded to put it back in the shelf :P
That got me wondering. For landlocked countries.. are rivers their only waste water outlet?