In the late 1980s, one of my elementary school teachers read my class a fantasy novel. I know that the book was decades old at the time, so I think it may have been written in the 1950s or 1960s. The author may have been British, but I'm not sure, and I have not been able to locate the book since.
The story began with a prologue featuring an immortal mermaid who owned a whistle. One day, she left it out on a rock when she was swimming, when a human appeared, took the whistle and vanished.
The action then shifted far into the future, probably hundreds of years, to a human viewpoint character (a teen-aged boy, I think). He lived in a world with roughly 18th or 19th century technology and no knowledge that there was still some magic in existence. He gets embroiled in some kind of political or military conflict while journeying across the countryside and eventually comes into possession of the mermaid's whistle (now refashioned as a key). Eventually, he learns more of the mystical history of his land and meets the mermaid (and probably some other magical creatures I don't remember). In the finale, he trades the whistle/key back to the mermaid in exchange for her help in destroying a dam; but although the dam does collapse, it is ambiguous whether the mermaid had actually lived up to her word or not.