http://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)
http://en.wikipedia.org/wiki/List_of_sunken_nuclear_submarin...
http://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)
http://en.wikipedia.org/wiki/List_of_sunken_nuclear_submarin...
As it turns out the ocean is also really good at both shielding radiation (water is what the nuclear experts use in large amounts, after all) and diluting radioactive contaminants, if it came to that, until the radionuclides themselves decay away and become stable.
Additionally it's not as if radioactivity is infectious or permanent. If something is radioactive here and I move it over there, it can't be radioactive in both places, which is why dilution is so effective.
Before you answer, you might want to compare "biological half-life" with "radiological half-life" (as they are usually far different), and why the same issue of bioaccumulation has noted caused famine from hazardous materials that we know to have been dumped into the ocean in orders of magnitude much larger quantities. How many oil spills have fisheries survived in general?