Ton for ton, I can see your point of how it's only a drop in the ocean, but if that drop is highly radioactive and spreads in the currents, wouldn't that be enough to cause serious damage?
I'm not sure what you mean here. Radioactive water is just water with some radioactive atoms dissolved in it. The total number of radioactive atoms in the million tons of radioactive water is fixed; they don't multiply. (In fact the number gradually decreases as the radioactive atoms decay.)
if that drop is highly radioactive and spreads in the currents, wouldn't that be enough to cause serious damage?
It will get diluted as it spreads; that was my point. The waters off the coast where Fukushima is are certainly not safe to fish in right now; but that's because the radioactivity is concentrated in a much smaller volume of water.
By if you use that definition you yourself would be considered "radioactive" as you are always walking around carrying quite a few radioactive isotopes, including Carbon-14 (as made famous by "carbon dating" from geology) and Potassium-40.
As pdonis notes, radioactive contaminants dilute just as well as any other contaminant in water. Assuming it more-or-less evenly mixes within the Pacific Ocean, you could essentially dump all of Fukushima into that ocean and not see large changes in overall ocean radioactivity.