The second question is, what is the environmental impact of dumping it in the ocean? No matter how dangerous it is, the ocean is gigantic. So you want to know: are there environmental processes that concentrate or amplify it?
Later: I had to leave before writing this last part.
Obviously, all things being equal, you'd rather TEPCO keep spending money to keep this stuff out of the ocean. But are all things equal? Is it riskier to try to contain it?
This is the wrong question. The relevant question is "what is the political impact of dumping it in the ocean?" and the answer is "devastating".
No matter how scientifically sound the solution, "dump it all in the ocean" simply can't happen.
Headline: "State Government to dump contaminated water from nuclear plant in ocean"
Quote: "There may potentially be a risk to flora & fauna, but we don't know for sure."
That's all it takes to make the idea political suicide. I think that's obvious from a mile away, even if many of us here agree that diluting it in the ocean is likely to be environmentally and fiscally sound.
Since we're allowing impossible situations, it could also happen if we had a magic wand.
That's kind of a dangerous precedent to set. If we can dump radioactive water into it, why not oil?
It was quite popular for a while. (And - debatably - still is in some parts of the world.)
http://en.wikipedia.org/wiki/Ocean_disposal_of_radioactive_w...
At the higher level of 5MBq/L, they would need to dilute it into 3000km^3 of seawater in order to reach double background radiation levels, or a patch of the 6km deep Pacific abyssal plain that is 22km x 22km.