I did physics at school too! I got a B in my GCSEs, I have absolutely no clue what I am doing posting here......
But seriously, you do realise that the sea itself and the products beneath it's bed contain radioactivity, don't you? Such things as NORM (naturally occuring radioactive material) and LSA (low scale activity)
Now if we have to break containment on any of our production and seawater cooling pipelines offshore we need to be suited up and have somebody there to take a reading. If the reading is too high then the ends are sealed and the part being removed is quarantined. By quarantined I mean wrapped up in a bag and shipped back to shore to be destroyed.
Also....everything has a level of NORM, just not at a high enough level to cause any harm.
As far as my understanding goes, if there is radioactive contamination going into the sea, won't it just be soaked up and diluted right down to the point where it is of similar strength to the LSA and NORM that we encounter every day?
Excuse the kind of roundabout way that I have done this.....I'm cack at explaining what is in my melon!