Seto no Hanayome

When the son of a mafia boss almost drowns, he is rescued by a mermaid. However, the young girl reveals that the two of them will now be assassinated, due to underwater laws which were put in place to prevent any evidence of mermaids making it to the rest of the world. The only option for their survival is for them to get married to one another, though this action changes their lives and sets them on a journey unlike any other.