In 2071, north pole ice have melted and the land has been submerged. Humanity survived but it's divided into two species: the true humans, those who live in underwater cities, and the Dugons, those who chose to undergo genetic modification to live in the sea. Both species go to war to survive as long as the Dugons consider the humans good to eat.

To defend against the Dugons, the city of Mari Ramelle builds a new kind of weapon, the mermaid robot, Salacia. Because of the small size of the the pilot cabin, the robot is piloted by a girl and a boy, both 13 years old: Marin Yuuki and Ushio Mizushiro. The fight for survival is hard, and during the manga, the city builds more mermaid robots.
-from Nagai Again