Gakuen Sensuikan-tai: Mermaid Girls

"Trenchwalkers," extraterrestrial beings that walk on the ocean floor, suddenly landed in Antarctica. Their climate-changing actions caused the sea level to rise by 70 meters and submerged Tokyo and other major cities in Japan. Furthermore, they began converting the water on the earth into "nektar," water of Gods, and those who could not adapt to nektar faced a high probability of death…

In the midst of this desperate situation, the handful of girls who can adapt to the nektar are sent into battle in submarines, wearing special equipment to fight and to reclaim the Earth from the trenchwalkers…

(Source: Big Gangan, translated)