I seen this was on Netflix. It has one season (with 26 episodes). It tells of a boy who almost drowns but is saved by a mermaid. When a human sees a mermaids true form they have to die, or the mermaid has to die, or marry that Mermaid.
He agrees to marry the mermaid and this show tells of some of the problems that arise.....
The story of Seto no Hanayome revolves around a young teenage boy named Nagasumi Michishio. One day during his summer vacation at Seto Bay, Nagasumi is saved from drowning by a mermaid, Sun Seto. Under mermaid law, however, either the mermaid whose identity was revealed, or the human who saw the mermaid, must be executed. In an attempt to save both Nagasumi and Sun's lives, Sun's family, which is the head of a mermaid mafia group, reluctantly decides that the two are to be married. Sun's father Gōzaburō is enraged about his daughter's sudden marriage. Between Gōzaburō's constant attempts on his life and the madcap antics of a slew of antagonists, Nagasumi has a hard time transitioning into his new married lifestyle.