Soukai no Seiki -Ouji to Otome to Kaientai-

In an alternate historical timeline where Sakamoto Ryoma survives his 1867 assassination, he reforms the government and establishes the Naval Support Division. Rinoie Narumi, a young boy, meets Saitani, a naval captain who leaves a strong impression on him. By 1914, Narumi enrolls in the Japan Naval Academy and is assigned to the Mikasa, a ship repurposed for training. Reuniting with Saitani aboard the Mikasa, Narumi chooses to remain there rather than continue his studies at the academy.