This Gilded Abyss

Sergeant Nix Marr wants to bury her haunted past in the bioluminescent ocean, but her loathed ex, Subarch Kessandra, forces her to investigate a massacre. Aboard a luxurious submersible, Nix discovers Kessandra's true motive: a violent illness is spreading, turning victims into killers. When another royal is murdered, Nix and Kessandra realize no one on board is safe, forcing them to rely on each other despite their bitter history.