Gakuen Sensuikan-tai: Mermaid Girls

Trenchwalkers, alien entities that traverse the ocean floor, appeared in Antarctica and altered the climate, raising sea levels by 70 meters and flooding major Japanese cities like Tokyo. They also began transforming Earth's water into nektar, a divine substance lethal to those unable to adapt. In response, a small group of girls capable of withstanding nektar are deployed in submarines, equipped with specialized gear, to combat the Trenchwalkers and attempt to reclaim the planet.