Nagi-Asu: A Lull in the Sea (2013)

Long ago, humanity lived in the sea, but some humans longed to go to land and eventually left the sea behind. Now in present day, the two human types co-exist though their thinking has become as separate as they places they have lived. When their school in the sea is closed, Sea Dwellers Hikari Sakishima, Manaka Mukaido and their friends have no choice but to attend the school on land but due to a special encounter, their lives gain a new twist.