Seto no Hanayome

Seto no Hannayome is a japanese anime released within Japan in 2007. It was a 26 episode series with an OAV in production. It tells the tale of Nagasumi, a young teenager on holiday with his parents, who gets pulled under water by a current and begins to drown. He is rescued by San, a mermaid. Because Nagasumi saw San's true form, they must either marry or both be killed in accordance with Mermaid Law. Whilst the 2 are happy to oblige to the marriage, San's father deeply objects.