My Bride is a Mermaid

瀬戸の花嫁

Nagasumi Michishio nearly drowns on a visit to his grandmother's seaside town, and the mermaid who saves him, Sun Seto, breaks her people's oldest law by letting a human see her true form. Mermaid law says that mistake means someone has to die, unless the two of them marry instead, which is exactly what Sun's family decides to do. Nagasumi ends up living under her parents' roof while pretending, badly, to be an ordinary high schooler with an ordinary wife. This manga is the original source for the multiple anime seasons and OVAs that followed.