Seems to be about Kaitens. To brighten our day, let us read the death poem from a Japanese submariner about to die. "This brave man, so filled with love for his country that he finds it difficult to die, is calling out to his friends and about to die."
Interesting fact, in the manga the mention of the word kaiten is what leave her crying.
Naturally - Kaitens are basically manned suicide torpedoes. Basically Kamikaze submarines with no escape hatch. Technically, the initial batch had an escape hatch that the pilot was supposed to exit through just before the final acceleration to target. None of the known pilots ever used that hatch and therefore the hatch was omitted from the later designs, turning it into a coffin laden with explosives. Once you're in a Kaiten, there's no getting out alive.
They're proof of what horrors a cornered nation will inflict upon itself rather than surrender. Not helped by the fact the military was controlled by hardliners who value Honor Before Reason and that all life is expendable.