Muromi-san

Fuente: AniList

Mukoujima Takurou is a lonely teenager who spends his time fishing at the pier and, to his incredible surprise, fishes up Muromi, a mermaid. Muromi first off doesn't realize she's a mermaid until she meets Takurou. Not only that, she is incredibly dense and crazy and has a drinking problem to top it off. Now every time Takurou goes fishing, Muromi appears and makes life interesting for him.