A Sinner of the Deep Sea manga volume 3 features story and art by Akihito Tomi.

Sinner Jo and boarder guard Akira make an unlikely team—but now their shared suspicion of lord Harujion's “Mermaid Island Operation” has brought them together for a daring mission! Sneaking into lord Harujion's residence, they will risk it all to discover the truth of his wicket plans… Ryuu, who has let herself become a pawn in the lord's scemes; Jo, who leads the rebellion against him; and Yuki, the human who is tossed in the turbulent waters between them… Where will they all lay when this tsunami of a story crashes into its conclusion!?