Since 1955, the US Armed Forces has know exactly how bad things can get. It took 60 years to declassify this document.
Quote (emphasis added) “Page 59. The problem of radioactive particles falling into the ocean raises the question of their availability to this portion of the biosphere. Plankton normally found in sea water are consumed in large quantities by fish.
These plankton concentrate mineral elements from the water, and it has been found that radioactivity may be concentrated (Page 60) in this manner by as much as a thousand fold. Thus, for example, one gram of plankton could contain a thousand times as much radioactivity as a gram of water adjacent to it. The radioactivity from these plankton which form a portion of fish diet tends to concentrate in the liver of the fish, and, if sufficiently high levels of contamination are encountered, could have a marked effect upon the ecology of an ocean area.
hat tip to Nuclear History for this story