While water is indeed a fantastic radiation shield, tritium is a radioactive isotope of hydrogen and quite capable of replacing normal hydrogen in a water molecule, so even pure water can absolutely be radioactive.

It’s one of the least dangerous radioisotopes out there, so I’m not particularly fussed, but it is incorrect to say water can’t be radioactive.

In this case, the water is radioactive. They are talking about releasing water in which a small fraction of the molecules are composed of Hydrogen, Tritium and Oxygen (HTO), rather than ordinary H20. HTO acts chemically just like H20, and is literally radioactive water.

> Water cannot be radioactive

To start, seawater has more things than water. Both Hydrogen and Oxygen have isotopes that can decay and emit radioactivity, so yes, water can definitely be radioactive, by definition. For a relatively short time because is not plutonium of course, but it can be.

And, unlike Plutonium, the problem is that life beings are mostly made of water and we didn't coevolved with a such rare element as tritium is in nature. Water is freely allowed to enter and travel and has granted access to every part of the human body. We trade a big danger for other, but this does not mean that radioactive water can't harm us.

I invite you to take a swim in it.