This is from Bloomberg news, about what a scientist from Woods Hole Oceanographic Institute predicts, based on the fact that the scariest substances escaping from the damaged reactors are soluble:
"Ocean currents and natural dilution of seawater contaminated by the crippled Fukushima No. 1 nuclear power plant are likely to spare marine life and the underwater ecosystem from devastation, scientists say.
Radioactive materials, including iodine-131, cesium-134 and cesium-137 have been detected near the southern discharge canal from the nuclear plant.
The ocean can absorb significant increases in cesium and iodine, the two most common radioactive isotopes coming from the plant, before it becomes unsafe for humans or marine animals, said Ken Buesseler, senior scientist at Woods Hole Oceanographic Institute in Massachusetts. Health authorities would still be wise to monitor seafood, seaweed and other ocean products, he said."