While there is quite a large possibility that it is in indeed a hoax, everypony needs to understand that we've been seeping thousands upon hundreds of thousands of pollution and (possibly) toxic waste into the ocean for as long as factory production has been around, or even before that.
That being said, if this weren't a hoax, radiation comes in a ton of forms already that aren't from nuclear waste. In California's bay area alone there is so much mercury in the water that we're advised to not eat anything we catch in the Bay, I mean people still do it, but we're told not to. I feel like it would probably be the same for if there was nuclear waste in the water as well.
I'm not a scientist, so of course I can't vouch for anything on this one. The best I can do is tell people/ponies not to worry and hope that this is just a hoax.