It still affects the immediate area. And Japan are big on fishing. So how far are their fleets going to go out to sea now?
I think people think the ocean is fair game but recently its ecosystem has been brought to light by coral decline and its links to holding back carbon. We have just had a big Netflix film that explains that the Ocean will have no major fish in it by 2048 and that will have an ecosystem collapse that affects all major problems we face today and you say it's ok to dump radioactivity in the Pacific, an ocean that you share and where the Thermohaline circulation pushes all that radioactivity towards the East Coast of America? It is not OK. And if you buy and eat Asian fish it really isn't ok. I don't know if the Japanese have exhausted all avenues and could only come up with this or not, but the idea that this is ok or that we will have no repercussions from polluting the ocean because it is vast is someone who doesn't understand that nuclear waste takes centuries to decay and will spread around the world via currents in that time.
You get radiation every second of every day. It's a question of how much. That won't harm even the locals. If it was airborne, that would be different, but still a local problem.
The Brits used to grind up their nuclear waste and pump it all into the ocean. The locals started getting all sorts of cancers and other diseases. That concentration was thousands of times higher than this will be, and went on for years.
There is a medical study in Japan, it tracks the health of people that got exposed to radiation by the nuclear blasts in WW2. Last I knew, it was still going, meaning some were still alive, all these years later.
Not saying that's a good thing, just that people get overly wound up about some things, and this is one of them.