It's worth noting that, as long as they don't explode (or get stolen), a lost nuclear warhead is pretty benign. They contain a relatively small amount (a few dozen pounds) of low-activity nuclear material and it all tends to stay together.
A reactor contains way more material, probably several tons at least. If it's been operating, it contains lots of nasty, highly-radioactive fission and decay products. Reactors also like to go all melty and sometimes (non-nuclear) explodey when they fail, although I suspect that's less of a factor when it's underwater.
Just for clarification its not anywhere near Miami (that is a small map graphic). It is right off the coast of Savannah, GA. There have been numerous books written about it and there is a wikipedia page about it.
