Kiso was sunk during the WWII, while a lot of Maru-yu's survived.

And it seems, that the author came up with a story, based on the mixed lyrics of the army and navy versions of this ballad: Maru-yu was somehow originally a very girly girl who loved Kiso, but was not allowed to get recruited. So to follow the trace of her love, she cut off her hair and joined the army as a Maru-yu. But in the end, Kiso was sunk before her