Mine is the breakdown of plastics over time, especially Styrofoam type materials. When they've broken down to microscopic sizes and become too numerous, the filter feeders at the bottom of the food chain may ingest enough of them to starve to death with full bellies. That will affect the rest of sea life. Yours is worth consideration as well, but technology will eventually be created for cleaner burning engines and reduce the problem.
They recently found a microbe that is happy eating plastic. Let’s hope it’s not too successful, or we will find ourselves fighting against a plastic-eating scourge.