My Bride Is A Mermaid

My Bride Is A mermaid (Seto no hanayome) is 26 episode Japanese anime featuring a mermaid Sun Seto.

Plot
A teenage boy named Nagasumi Michishio is saved from drowning by a mermaid named Sun Seto. According to mermaid law, if a mermaid is seen by a human, then either the human who saw the mermaid or the mermaid who was seen must be killed. However, there is a loophole where neither have to be killed if the human and mermaid marry. As a result, a marriage between the two is arranged by Sun's family, which is the head of a Yakuza group. Sun's father Gōzaburō is the head of the family, however, and he does not approve of Nagasumi and treats him with hostility. Nagasumi must learn to adapt to his new life with his bride Sun while surviving a barrage of assassination attempts that are orchestrated by her father.