All the plutonium and uranium in those reactors was mined from the planet earth, it was already in the earth, only greatly dispersed, it will be even more dispersed in the ocean. radioactive Uranium IS a natural substance. People have been bathing in radioactive hotsprings for along time, believing them to be healthy.

the reason CESIUM is deadly is because it chelates potassium in the human body, there is so much potassium in the ocean water it would be insignificant.