It's nothing to do with maritime fuels - it's because the sea absorbs a lot of the temperature from the air. This means the air temperature stays lower, but affects marine life.
It has happened as a direct result of climate change.
Ok, to summarise, more heat is going doesn't into the oceans because the pollution isn't reflecting it. Got it. But sea temperature rise as a whole is still due to global warming and absorption of heat by the oceans. If the additional heat is being absorbed more by the sea than it was, that's bad. But it's bad anyway.
I've said my piece, I'll leave it to those more knowledgeable to argue further.