There are two things to keep in mind: dilution and buoyant nuclear debris. Robert Alvarez in a 2012 article (tinyurl.com/c3xkdhd) discusses how governments secretly knew in the '50s that dilution in the ocean is a myth; hotspots will remain. My particular worry is hotspots containing uranium and plutonium, which can float in the ocean column; researchers have found that they will become entrained in ocean mist and travel quite far. Inhalation of radioactive mist - even at low concentrations of plutonium or uranium - is extremely dangerous to health and can lead to lung cancer. Another concern is buoyant radioactive waste in the water - either from nuclear waste washed out to sea from the TEPCO site or other buoyant debris made severely radioactive by Fukushima plumes.