If everyone starts dumping their nuclear waste like this, then yes. Which is why this not cool and Japan should do the right thing and just pay for solving this properly and responsibly. They took on the moral responsibility to do that when they started planning to build the reactor. Tough luck it didn't work out for them with getting some electricity on the cheap. Just bad luck or bad planning (leaning towards the latter).

Nuclear waste is inconvenient because handling it properly is very costly and as soon as you start accounting for that, nuclear stops being cheap. Or put differently, it's only cheap if you make the waste somebody else's problem. Not unlike people's tail emissions from their cars are somehow not their problem but we all get to enjoy the effects of global warming.

It's more a question of "can Japan get away with just dumping nuclear waste in international waters?". Answer: yes of course they can. People will whine a bit but nobody is going to do much more than that. Probably some marine life will be affected. And there's a chance that shows up in our food chain. (Japanese love sushi, just saying). But the Pacific is a big place so the chance of that is not super high. Or it happens in a few hundred years when that crap is still radioactive. By then it's the problem of a few generations after us. We might simply call that bad luck and claim that will not have a measurable effect. A few people dying in the future would be hard to measure or connect to this specific event. That doesn't mean it won't happen but just that you have plausible deniability. People confuse that with "it's fine". Depends if we are talking legalities here or moral responsibility.