Moon Child(1989)

Will a half-mermaid, half-human girl bring an end to the feud between humanity and the mer-people — or bring about the destruction of all life on Earth? As a merman named Shona is returning to his birthplace to spawn, he meets a young human, Jimmy, who suffers from amnesia. Together, Jimmy and Shona set off to find the missing girl. Will they be able to avert a mysterious prophecy, or will finding the secret of Jimmy's identity delay them enough to set Earth on the path to destruction?

(Source: CMX)