The whole concept is nonsense. What's to prevent any flight into the US (whether a foreign carrier or even an American one) from transporting a radioactive device?
It wouldn't have to come by boat, and probably wouldn't in any event.
Shall we demand that Americans screen all onloaded baggage and cargo coming into America at each airport overseas?
Heck, we don't even use Americans to search the baggage in airports on American soil.
I think it would come by boat. It is easier to hide it in a cargo in a big boat than in a relatively small airplane.