So I guess the problem is that the water, instead of being just plain H2O, in a tiny percentile of the water the hydrogen has one or two extra neutrons making it have a high probability of radioactive decay. (about 3 grams of the 800000 cubic meters of water) Since chemically it acts like water in every single way, except that the hydrogen's extra neutron makes it a tiny bit heavier, that means that it is insanely hard to filter it out from the rest of the water. The neutrons have no special electric or magnetic charge to chemically interact with any filtering or chemical mechanism.

In distilled water, this decay wouldn't even be an issue, but in salt water there is a tiny chance that the decay can hit other atoms and create radioactive iodine (I think it's iodine?) that is poisonous to people. However, the truth is that the ocean and all people are constantly being bombarded with radioactive neutrons from the sun, and that all soil and water has a tiny percentile of material that is radioactive anyhow. So in this context, the water being released is radioactive, but the amount of radioactivity is basically background noise compared to everything we interact with on a daily basis anyhow. The ironic thing is that the radioactive exposure we get from trace elements burned in coal straight from the ground is more than we will ever get from nuclear, so it's a moot point anyhow.