Gate Season 2: Jieitai Kano Umi nite, Kaku Tatakaeri

On a certain summer day in the year 20XX, in broad daylight a gate to another world appears in Ginza, Tokyo. An army and a swarm of monsters come storming through the gate. The Japan Ground Self-Defense Force is able to defeat the invaders and they head through the gate into uncharted lands. An epic fantasy about a battle between the human race and another world filled with dangerous enemies and beasts! *Source: Alpha Manga*

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)