Umi Monogatari ~Anata ga Ite Kureta Koto~

Gentle Himeko and reserved Chikane were born into very different backgrounds, but still became the best of friends. Their fates, they would soon come to discover, are intertwined in a fashion that they can't possibly imagine -- they are the reincarnations of two priestesses who once sealed the god of darkness Orochi in their past lives. Set to revive again, his giant robot-riding agents have set out to destroy the girls, one of whom is Himeko's friend who is in love with her. Riddled with memories of their past lives, the two try to survive long enough to cast the seal on Orochi again...

While visiting his hometown, junior high school student Michishio Nagasumi's life was changed forever when he nearly drowned. Though he was saved by a mermaid named San, there’s a catch -- her family is part of a merfolk mafia, and they abide by a strict rule: If a mermaid is discovered by a man, either he or the mermaid must die. That is, unless the man marries into the family! Faced with such a decision, Nagasumi can only oblige their customs and become a "happy" newlywed. Now, with a new school year around the corner, Nagasumi must find a way to deal with his new bride, keep her true nature a secret, and most importantly, escape inevitable death at the hands of San's over-protective father’s gang!