Heart Fluttered by Deep Blue

Namelia, a royal mermaid warrior, disguises as human bodyguard Selina to monitor tycoon Elliot, whose deep-sea drilling project will destroy her underwater homeland. The queen orders her to kill him if the plan proceeds. While protecting Elliot from assassins and a hostile supernatural group called Abyss, she gradually falls for him and learns his father's death was caused by corporate blasting, not merfolk. The pair team up to stop the drilling scheme and defeat the traitor mermaid Deep Lord. They overcome the rift between humans and merfolk, hold an undersea wedding, and raise a mixed-blood daughter who bridges two worlds, living peacefully by the sea.