There was (and maybe still is) a project to build an open-source low-power autonomous robot to clean up oil spills. https://sites.google.com/a/opensailing.net/p

There was (and maybe still is) a project to build an open-source low-power autonomous robot to clean up oil spills. https://sites.google.com/a/opensailing.net/protei/ https://sites.google.com/a/opensailing.net/protei/ Not sure if still active. Presumably, you could build something to pick up trash in general. Bigger issue though is the trawling -- junk or no junk, trawling is incredibly devastating to the ecosystem.