A Lull in the Sea (Nagi-Asu: Nagi no Asukara)

Since long ago, human civilization had lived on the ocean floor. However, there were many humans who wanted to live above the surface and they moved to land creating a fundamental separation between the two. After their school closes down, four 14-year-old middle school students from the sea village, Shioshishio, have to attend Mihama Middle School on the surface. What follows is their struggles to adjust to a new environment and the relationships between the sea and land people, while dealing with their own newfound feelings that have just started appearing with the end of their childhood.

(Source: Wikipedia)