For this to be feasible you'd need a deepwater port with dense on-shore infrastructure. Most nuclear plants are near bodies of water but the depth is pretty shallow, so you perhaps couldn't float something like this up very close.
On the other hand, there may be other things out there of a much softer variety. One practical, real-world application of Homeland Security resources would be routine interdiction of port and river traffic near these areas. For example, the Bayway refinery in NJ, or some of those plants along the Houston ship channel...?
My thoughts exactly. I've seen the area, and it has lots and lots of chemical plants. The dollar cost, if the area were destroyed, would be high. And the environmental costs would be dreadful.