This is a real-world technology that found its way into the series. The basic idea behind the supercavitation propulsion is to reduce the friction of the object with the surrounding water by covering it with a stream of small bubbles. However, these very same bubbles also drastically reduce the effectiveness of standard propellers and, thus, rocket engines need to be used instead.
According to the history of the "Aoki Hagane no Arpeggio" universe, supercavitation propulsion became widely used as a means of increasing the effectiveness of torpedoes in 1990s. This in turn led to a great evolutionary 'explosion' of the ships' designs as new means to counter the threat of super-fast torpedoes was needed. And while this technology is indeed quite effective at increasing the chances that the torpedo will reach the intended target, it does nothing to make the payload itself more dangerous. Thus, the torpedoes using the supercavitation propulsion were just as useless against the "Fleet of Fog" vessels prior and during the Great Battle, as any other human weapons.
Sometime in the first half of 2050s, the idea to create offensively-powerful submarines that would eventually be able to combat the "Fleet of Fog" was born in the minds of the Japanese engineers. To give these submarines the protection needed to survive the fight, while still having them as cheap as possible, the modular design (so that the damaged parts could be easily replaced) was proposed. That alone, though, would never be enough, and so these submarines were also given the means to shake of the "Fleet of Fog" torpedoes and anti-submarine missiles: the supercavitation propulsion. This, the Hakugei class submarines were born. While two first vessels didn't survive their encounters with the "Fleet of Fog", Hakugei (3) did and actually managed to land a killing blow on two battleships.
- Not only the supercavitation propulsion is the real-life technology, but, according the afterwords to the first volume of the manga, it was something around which the whole series were built: fast and super-dangerous torpedoes and cute girls. That is what was the cornerstone of "Aoki Hagane no Arpeggio" manga.
- Both humans and the "Fleet of Fog" make use of this technology.