The release of radioactive material has been estimated to be between 500 PBq and 1000 PBq
The Pacific Ocean covers 165 million km^2 and contains 66 million km^3 of water.
Assuming all of these 1000 PBq (10^18 Bq) to be equally distributed over 1 % of the ocean to a depth of 50 m, and without considering the ‘sinking down’ of heavy material, you will get 12 Bq/kg. If you thereafter consider that some 90 % of the radioactive release is Iodine131 (see page 116 in UNSCEAR 2013 Report,) only some 2 Bq/kg will be active at the time of the horror-stories.
Further, it may be worth mentioning that the US National Academy of Science has referred to measurements showing 7 Bq/m^3 Fukushima-derived fallout of 137Cs near the Canadian continental shelf.
This may have been rather difficult to measure, considering that the natural level of radiation in the ocean is 11,000 Bq/m^3

In order to relate to something familiar, it can be mentioned that all of us carry with us about 65 Bq/kg (65,000 Bq/m^3) as part of our human body, or that a radon level of 100,000 Bq/m^3 was found in the basement of an occupied house. (Watras Incident, as discussed on another post.)
Correct me if I am wrong.
Fukushima Greenpeace
Considering this, it is difficult to understand how well established “green organizations” show maps and indicate the “extreme danger” related to the radioactive pollution the whole way to the American west coast.
1 RAD = 10 mSv However, this unit is (deliberately?) confusing and will not have any meaning without stating the time. E.g. RAD/hour.

Without being able to check, I think this link from The Register is more reliable.

If you have trust in World Nuclear, the following may be interesting:

  • A silt fence has long been in place to prevent contamination reaching the open sea and the diluting effects of ocean currents mean that radioactivity cannot be detected in seawater beyond the plant harbour.

If you are a fan of YouTube and have seen “The Ocean of Death”, you may wonder how the many fish can be so sensitive that the increase in radiation from 11,000 to 11,007 Bq/m^3 has caused the disaster shown on the film.
Contrary to radiation; fish are rather sensitive to change in the concentration of salt in the water. So, if not directly falsified, the dead fish may be found outside a plant for desalinating seawater.
Besides this it is worth looking at the last table in a paper from UNSCEAR. Here it is seen that the radiologic tolerance  for fish is some ten times that for mammals and birds.
On the other hand you may be relieved to see how the animals are thriving in the highly radioactive and forbidden zone at Chernobyl.

As far as I am aware of, Tepco’s limits for groundwater contamination is that groundwater should contain less than 5 Bq/L of beta ray-emitting radioactive material and 1 Bq/L of caesium-134 and caesium-137.
Again, this should be seen in relation to the natural load of the human body: 65 Bq/L and of the ocean: 11 Bq/L
Natural Radiation EU
To be on the safe side, the standard for public radiation exposure was (arbitrary) set
to one millisievert/year.
This is only one tenth of what some Europeans have been exposed to throughout their lives.

My conclusion is clear.
Something is rotten – somewhere –
But where ?