If you go to the Los Alamos Science Museum you can see an original letter from Albert Einstein where he proposed that a nuclear bomb could be fit on a ship and sailed into harbor in 1938. A ship-sized nuclear bomb was not of interest to President Roosevelt at the time for a likely good reason. It's only when the UK realized that it wasn't a ship size. It was something that you could drop from a conventional airplane bomber of the day that the US became interested.
It was also a failure of imagination. A ship sized tsar bomba at the time would still be quite interesting - armor the ship like a Iowa class (or more), and give it an automated navigator (very doable at the time, plus or minus a mile or two), and it’s a war ender.
Seen it, read it, and aware of why it was ignored at the time - the ability to make such a weapon, as relayed by Einstein, was a hearsay tale of speculation.
Some time later Mark Oliphant visited the US in person and walked key people through a circles and arrows on chalkboard breakdown of how such a thing would be actually possible.