Seto No Hanayome漱户之花嫁 ; 瀬戸の花嫁 ; Inland Sea Bride ; My Bride is a Mermaid ; 세토의 신부

From AnimeWaves: A young boy gets saved from drowning by a mermaid but according to mermaid law, if a human sees a mermaid's true form, both are to be executed. The only solution to the problem is for him to marry her and become part of the mermaid family.