18+ Soukai no Seiki: Ouji to Otome to Kaientai

In another timeline in history, one where Sakamoto Ryouma was not assassinated in 1867, Ryouma successfully overthrows the government and creates the Naval Support Division...

Two years ago, Rinoie Narumi, a young boy, met Saitani, a beautiful naval captain who immediately captured his heart. It is now 1914 and Narumi has enlisted in the Japan Naval Academy and is stationed on the Mikasa, a ship that is now being used for training students. He finally gets reunited with Saitani and decides to stay on Mikasa instead of continuing with the Naval Academy.

(Source: MU)