The short answer is that no one knows.
Much has been written recently about the radioactive ocean-water plume that is due to hit the west coast sometime this year, possibly as soon as next month. The problem is that although we have models that predicts its arrival, we don't know exactly what the amount of radiation will be or where along the coast the radiation will end up.
Higher-than-normal levels of radiation have already been detected in tuna and kelp in various places along the North America west coast, from Canada to southern California, most likely as a result of radiation contamination brought from Fukushima by air currents. But a big blast from the destroyed nuclear plant is arriving by sea now.
It's estimated that we will receive higher levels of radiation for 6 - 9 years, but that estimate may be based on the single event of the March 11, 2011 explosion. Radiation-contaminated water has been leaking continuously from the damaged plant since the explosion. And Tepco, the owner of the Fukushima plant, is planning to dump all of the radioactive water still housed there into the ocean, at some point.
How much exposure are we receiving? How bad is it going to get? What can we do to protect ourselves? These questions have not been satisfactorily answered. Conspiracy theories abound, people are taking their own Geiger counters to beaches to test for radioactivity, blogs are written predicting one million future deaths from radiation, articles are written counseling calm in the face of uncertainty.
What's lacking is an ocean-water testing program by the federal government. The reason for this is that, unlike airborne radiation testing, testing for radioactivity in ocean water is not covered by any existing government agency! This has essentially fallen through the regulatory cracks. One ocean-radiation scientist, Ken Buessler, has taken it upon himself to do the testing. He is asking people to donate to his efforts. Another scientist is planning to test kelp in the waters off the California coast.
It's time to put pressure on our public officials to fund a testing program!
Other links of interest: