Unjust Depths by Madiha S.

A thousand meters under the ocean, beneath an uninhabitable surface, the communist Union of Ferris, Lyser and Solstice scratches out a living for its people in former penal colonies won in a revolution against the vast Imbrian Empire. With the Empire and locked into war with the far away Republic of Alayze, the Union bides its time, hoping to be able to defend their way of life if the Empire returns.

Out of every junior officer in the Union navy, Lieutenant Murati Nakara has made the most petitions for promotion to an active duty warship. When the Empire enters an era of internal chaos, Murati finally gets a chance to prove herself — and gets her wish to serve. Aboard an experimental ship and part of a young crew inexperienced in real war, Murati will journey across thousands of kilometers of ocean, discovering unlikely allies, dangerous foes, and a myriad truths about their life under the ocean..