The submarine nickname Jimmy has competing origin stories

Where did the nickname Jimmy, heard in a submarine film, come from?

It is long-standing submarine usage, but its precise origin remains unresolved.

Jimmy is treated as a long-standing nickname in submarine usage. Several competing stories circulate about its origin, with no firm etymology established here. The historical specialists consulted are described as disagreeing. The film prompt is unclear in the captions, so it does not establish exactly whom the nickname identifies.