That's an interesting link, and point; but my understanding it the water is contaminated.
So is the water in the story for the link. The problem is that contaminated is not a binary function. If the material is sufficiently dispersed in the water radiation levels won't rise much (I say rise because sea water is already naturally radioactive, as most things are. Which one could say that sea water is "contaminated" but that wouldn't be useful). If it isn't sufficiently distributed then the distance between you/a fish/sea creature is going to play a big factor since water is really good at stopping radiation. Basically think about you and your wifi router. If there's a metal wall between you and the router you're not getting a signal. Attenuation is the key concept in both radiation and the wifi router. Though this is obviously the fall back for "if they mess up" which isn't likely.
'Chuck it in the sea' has been the answer proposed by many industry scientists for a long time now.
Even supposed small changes are being seen to have an effect on ecologies and food chains [1]
[1] https://e360.yale.edu/features/radioactivity_in_the_ocean_di...
So a bunch of nuclear scientists did a calculation and the results sound scary. And Korea is using it as a political tool. Japan and Korea are at each other all the time, though this may not be common knowledge to Westerners. But if you need a sniff test for this to pass, remember that you can go swimming in a nuclear reactor and people actually do, as many may have learned from the XKCD What If[0]. So just take that story and add a ton more water and add convection so the distribution becomes far less homogeneous. It should pass your sniff test now.
[0] https://what-if.xkcd.com/29/