I am not a “nukear” engineer, but I do not understand how
radioactive water from Japan, mixed into the Pacific Ocean
could have any significance 6000 miles away.
Bioaccumulation.
You are correct. You could take all the nuclear waste that mankind has produced since Hiroshima, dilute it in just one cubic mile of seawater, and safely drink a glass of that water every day for the rest of your life. And the Pacific Ocean contains over a hundred million cubic miles of water.