Ao no 6-gou, Aoki Hagane no Arpeggio: Ars Nova Recommendations
The same plot where mankind's existence lay on the hand of submariners who are up against weird looking enemies, however one is limited to a 4 ep OVA, while the other one is a full TV anime.
On the surface, these anime are similar since they are about submariners fighting for humanity against lifeforms that rose from the sea, and they both provide the thrill of naval warfare with plenty of excellently-animated submarine scenes. But at a deeper level they are really about the enemies, born into the world with intelligence rivalling that of humans, as they struggle to understand their newfound emotions and find a moral way forward. Aoki Hagane was surely inspired by Ao no 6-gou, but offers its own take on these themes.