Good question. It’s not exactly water soluble, so how is the uptake less on land rather than at sea?
Is it air dispersal after incineration of waste, then precipitation into the sea? I don’t know.
It will form compounds, and no doubt form easier bioavailable routes into certain parts of the aquatic food chain. And build up.
Fish can’t escape the ocean, that’s their environment.
Pretty much like we can’t escape the atmosphere.
I would suggest that it might be because mercury pollution is going directly to the sea rather than land areas…or at least leaching from land dumps into the sea. Or through precipitation.
It’s nothing I know much about- apart from avoiding eating too much of the fish that can accumulate too much mercury.