Aoki Hagane no Arpeggio

By the early 21st century, most of Earth's land has been lost to rising sea levels. The Fleet of Fog, a group of sentient alien battleships of unknown origin, overwhelms Earth's naval forces, pushing humanity toward extinction. In 2056, Captain Chihaya Gunzou and his crew aboard the Blue Steel fleet represent a final effort to resist the invaders. The Blue Steel fleet possesses a unique asset: a mysterious I-401 submarine capable of transforming into a girl.