Ningyo-kun, Oyome ni Mairimasu

Takasaki Akihiro works as a high school teacher on a small island. On his way home, he meets a strange young man, who says that he was a mermaid whom Akihiro had saved a long time ago and was there to return the favour. To top things off, the young man proposes to Akihiro, saying, "Please make me your bride!"

Not wanting to involve himself in anything troublesome, Akihiro tries to get the him to return to the ocean. However, after being earnestly told "I've always liked you" and seeing him admirably perform the household chores to become his bride...?

The devotion and love between an overserious high school teacher and an admirable and earnest mermaid.

(Source: Gentosha, translated)

Note: Chapter count includes an epilogue.