Otohime Diver

Sixteen years ago a giant earthquake caused what was called "the great immersion" which submerged most of the continent, and since then people coexist with the ocean.

Minato Tatsumiya, a young student at Enoshima Oceanographic School for Girls, will attempt to subsist in this world full of mysteries.

Links: Official English (Published by VAST Visual) - Azuki, MangaPlaza