Ryuuguuchou wa Umi no Soko

Ryugu is a small town under the sea. Aside from the fact that everything is, well, underwater, it's just like every other small town. Classmates Masora and Sumire are like every other middle school girls, swapping love stories and hanging out in shopping malls. But for some reason, Sumire feels like this "ordinary life" is all going to come crashing down... (Source: Manga Planet)