Aoki Hagane no Arpeggio: Ars Nova

In the year 2039, the drastic effects of global warming have caused sea levels to rise, resulting in major loss of land. To ensure humanity learns its lesson, a fleet of powerful warships equipped with advanced weaponry emerges, causing overwhelming defeat for naval forces throughout the world. Calling themselves the Fleet of Fog, each ship has a "Mental Model," a humanoid avatar which represents the ship's heart, its "Union Core." Each model has a different personality but all follow an unknown authority known as the Admiralty Code, who have implemented a blockade to prevent humans from traveling across the sea. Seventeen years after the blockade, Gunzou Chihaya, a National Marine Academy alumnus, stumbles upon I-401 and its Mental Model, Iona, who sides with mankind for unknown reasons. Utilizing this newfound hope of achieving peace for humanity, Gunzou and his friends form a group of privateers, known as the Blue Steel, as they set out on a dangerous journey. In the story, a Fleet of Fog loaded with super weapons suddenly appear all over the world. Without the ability to withstand this fleet, humanity was defeated and could no longer travel the seas. 17 years after the devastating naval war, Chihaya Gunzou and his friends somehow commandeer a Fleet of Fog submarine and rename it I-401. Together with Iona, they take on the Fleet of Fog.