The tragic story of Lorelei. A legend retold with a splash of Sci-fi.
Lorelei is more monster than mermaid. Cursed and bound to a boulder in the middle of a river, she waits. She's been waiting for a long time. When a trawler boat makes a wrong turn, she chokes on a song that's older than the wind. A storm descends on the river and tears everyone apart. No one can hear her mournful song now.
Except for young Henry. The last man alive from the trawler. He sees a mermaid and meets a monster.
Note that country isn't specified. Although I kept in mind the Rhine river in Germany while writing, since the script is one scene, it could be set anywhere.