Gakuen Sensui Kantai: Mermaid Girls

In the manga's story, extra-terrestrial life known as Trench Walkers reveal themselves, emerging from the Antarctic, and walking on the sea floor. Their goal is to terraform the Earth to their liking, which involves raising the sea level by 70 meters, thus rendering Tokyo and many other population centers uninhabitable. They have also begun to change Earth's surface water into what they call Nectar, the "divine water." Those who are incompatible with Nectar have a high chance of death when imbibing it. However, there are girls who are found to be compatible with Nectar, and they are recruited to be marines in a fight against the Trench Walkers, boarding ships to do battle in a new deep sea front line. The story centers on Hiro Todoroki, who receives a message from an older sister, a marine supposedly missing in action. The message tells Hiro to find a formidable marine named Kiriri Tsunoba. Though lost and confused, Hiro heads to the Masakado Girls Academy Submarine Unit with the intent to join them. (Source: ANN)