Perhaps some "home work" on where there is already agreement. I believe 87 countries have signed on to this... https://www.epa.gov/ocean-dumping/ocean-dumping-international-treaties
Finding where agreement already exists, drawing more into the agreement, then -- expanding the agreement to resolve more issues.

Sometimes it seems we come to the conversation late, and then invest a huge amount of energy 'reinventing the wheel'. A larger issue is the enforcement of such agreements. Ocean policing? International waters begin so far from the shore that a patchwork of coastal regulations bring us back to self-policing. Help me to identify my  contribution to the problem. Bottom up solutions often lead to a global change of heart.