Shipping doesn’t work that way, all ships are prescreened prior to arrival at other ports. Additionally, a nuke of small size would not destroy an area much bigger than the Port of LA. Not saying it wouldn’t be bad but it would not be destructive to the city of LA.
Any ship bearing even an atomic weapon rather than a powerful hydrogen bomb which is detonated, for example, in New York Harbor close to Manhattan will kill an enormous amount of people.
Where are these ships prescreened? At the port of departure? If so it seems to be a practical possibility to arrange a rendezvous at sea with a fishing smack bearing a bomb and transfer it to a freighter bound for New York City.
If the pre-screening is done at sea it would be more effective but it seems to me hardly feasible in terms of manpower and technology.
