So about the containers for radioactive waste: to use Germany as an example, there are mutliple(!) issues with the simple containers used in the interim storage site Asse, where they simply corroded and started leaking radioactive material. So "normal" containers just don't cut it. To effectively seal and shield so-called "High-level radioactive waste" (which is basically the used fuel rods from a nuclear power plant) the CASTOR containers are used. Those reduce the radiation to some extent - but still not enough for a human to be able to stand next to them without issue. And that is not taking into account that the fuel rods are HOT. As in thermally. (This is btw how you generate heat in a nuclear power plant - you just use the heat from the fuel rods to boil water.)
  To sum up: you have insanely thick steel (or copper) containers which are super hot. And big. And made from metal, which enjoys corroding in salt water.

  And like in Tschornobyl, used fuel rods can kill you with their radiation in a couple of minutes if you just stand close enough. Diluting something like this would obviously reduce the immediate danger, but then secondary radiation effects kick in which basically means an increase in cancer rate. So if you throw something like this in the sea, you would probably kill any sea life around (not to mention you would also boil the water probably) it and give cancer to the rest. And since the radioactive particles are now in the fish, which humans tend to eat a lot of, now pretty much all humans have it too.