Seto no Hanayome  

My Bride is a Mermaid, SetoYome, The Inland Sea Bride, 瀬戸の花嫁

A young boy was rescued from drowning by a mermaid. However, as per mermaid law, if a human witnesses their true form, both of them must be killed. The only way to resolve this issue is for the boy to marry the mermaid. But can they keep her identity hidden from being exposed?