Install information boards at beaches about aquaculture (and fisheries) waste (how it does look like). Motivate the public to collect this kind of waste and to bring it to a municipal central location or directly to the owner. In parallel set up a sea-basin or international registration system to report waste: An easy to access online platform should give people information on the collected items (i.e. QR-code, number, etc.).
Relevant type of aquaculture:
Relevant sea basin: