Seems like she heard the news about Musashi having been found at the bottom of the sea and misunderstood it as one being able to become a battleship by diving into the sea.

Oh yeah, Kiyoshimo's one fine sub battleship now! Incredible! Powerful! lost in translation: the Commander said 'sensuikan' (submarine) with the 'sui' being inaudible, so it sounded like he said 'senkan' (battleship) instead.

Moshiyoki I wrote it in reverse intentionally.