So far, they probably haven't released enough radio-isotopes into the sea to cause serious damage.

Release a significant proportion of what they have on-site and it's bye-bye to the fishing industry in the entire Northern Pacific for centuries.

Look at the ocean current patterns.

There is a much weaker link to the southern Pacific, down the coast of the Americas and via the equatorial currents. That would have a more delayed but much more catastrophic effect - the anchovy fishery off Peru is used in animal feed worldwide. Dunno the figures now, but a few years back it accounted for half the landed weight of ALL fish caught anywhere, nearly all processed into fishmeal for animal feed. Eliminate that and a large fraction of the world's intensive animal rearing would go with it, including most of the American cattle industry.   It might take a century for a significant amount of Japanese nuclear pollution to get into the southern Pacific circulation, but that's the real biggie.