In the Black by Lyra Evans

Niko’s life is settling into a comfortable routine. Every morning he wakes to Cobalt, his gorgeous boyfriend, before spending the day trying to find answers to questions no one else is asking. With leeway from his Captain to investigate as he pleases, Niko somehow finds himself uncharacteristically happy. He should’ve known that wouldn’t last long.
The case he’s investigating is a missing person, and things get complicated fast. The victim was part of a secretive and reclusive group known as the Choir. Outsiders don’t have access to members, don’t even know where the Choir lives and operates. While one person tells Niko the victim disappeared under suspicious circumstances, everyone else seems to agree she just left on her own. But why would a young woman at the peak of her career just drop everything and leave? To find out the truth, Niko has to go where Cobalt planned never to return—Azure’s Court, deep in the ocean.
Playing as though he’s just curious about Cobalt’s former life as a Royal Guard, Niko questions Cobalt’s old friends. In doing so, he finds out much more than he bargained for about his lover’s past—including Cobalt’s ex. The more Niko uncovers, the more questions he has, both about the case and his relationship. He thinks finding out what happened to the victim will give him the answers he needs, but it might just force Niko to make the hardest choice of his life. What’s more important—justice or love?