Gate Season 2: Jieitai Kanoumi nite, Kaku Tatakaeri

Captain Gorou Edajima, a seasoned captain who has visited the various countries of the Special Region on a research mission, and master cook Petty Officer, 1st Class Hajime Tokushima were given a special mission: rescue an American journalist kidnapped in the Special Region! Aboard an Oyashio-class attack submarine, they set out onto the seas of the Special Region, but little did they know their mission would lead them headfirst into a whirlpool of petty rulers and pirates vying for control in the Blue Sea...

(Source: AlphaPolis, translated)