A few flaws..
If I were a terrorist I would see this as a very soft target for numerous and obvious reasons. To call it a tempting target would be an understatement.
As an eco nut I would be all over this as a disaster in the making.
First it would alter the surrounding eco-system by raising the water temperature. As an example, if this were put off the coast of Maine you would see a drop in the quantity and quality of the cold-water lobsters and cod caught in those waters.
Then you have the potential of a radioactive leak getting caught in the ocean currents and dragged all over the Atlantic. Think of Chernobyl happening under the sea. In Russia much of the serious radioactive dust fell in a relatively confined footprint down wind.
Using the Maine example again, you would be talking about the poisoning of ocean based food supplies from the east coast of the USA to Canada, Greenland, Iceland, England, Europe, Africa, etc? I think you get the picture.
Finally what do you do with the leftovers when it hits EOL? Do you abandon it in place as stated by teched? An undersea cleanup would be a challenge.
