Singing letter bomb - collect.
Sign here........x
Realistically - the most devastation from a rogue nuke would be a west coast detonation just a few miles off-shore - outside U. S. Teritorial limits.
The 'Other' countries have not taken the effort to refine thier fissionalbe material to the purity that we have, so anything used by those nations
would essentially be of a 'Dirty' variety, releasing tremendous amounts of radioactive fallout, manufactured during the thermonuclear event.
Salt water, the delivery vehicle itself, all other material within the CEP, or ground zero footprint, would be engulfed in the fireball, and rise to
an altitude easily exceeding 50,000 feet - well over 10 miles. Heavier particulates would fall upon the immediate coastline communities,
and the lighter materials would be distributed across the country by perdominate weather patterns, and would eventually wrap the planet
several times, with more debris coming down and lighter elements falling out over expanding areas for months.
Half-lifes of some these materials go into the 20,000+ years, so effects would linger for 100,000+ years.
With the shipping lanes throughout the world, and the cargo volumes being moved everyday finging one such vessel
would be more luck than not, as there are ways to shield the source from detection if it is in fact a sophositcated device.
Delivery method need not be fast, just deliberate. They have all the time in the world on their side.