What is the half life of radioactive water?

I just asked what is radioactive water and I got the answer. But I now realise what I needed to know was more about the dangers ociated with it. Once radioactive water is released in the ocean, how far is the radioactivity lost? What is it's half life? Because actually it looks like not a bad idea at all, it might diffuse the radiation and make it safe for everybody. Or am I totally wrong.