There are lots of folks working on maritime (and aviation) decarbonization, it's not being ignored. It's just harder than decarbonozing other sectors because they can't just electrify like you and I can do with our cars and homes. The solution is likely to be synthetic fuels of some sort, ammonia, hydrogen, biodiesel, etc. We're seeing sails come back, there have been innovation hull designs, etc. You could even call tarrifs a partial solution here because building locally reduces shipping needs. It's just not as cheap/easy as installing solar panels/wind/batteries though. We need policy to drive change here, which puts it on a different level than the personal responsibility measures. I absolutely agree we need to do all of the above though.

As to the source, I don't know but it's cited in government records everywhere. They have a good handle of how much fuel is produced everywhere, we know exactly what ships exist and where they go in real time globally, we know how efficient they are, so it doesn't seem nebulous enough to me to have any real doubt in. NASA can probably track all their emissions from space too.