Maidens of the Sea
This little indie film Maidens of the Sea features a man who learns about and travels to an island that seems to have an island resort ("seems to" in the sense that it's unclear if the resort is open or has been closed to outsiders). While he's there he figures that there's something strange going on, which is eventually revealed to be the fact that mermaids occasionally visit the island and the people who live there keep their secret. Or something, it's not really clear.
Anyway, there are brief shots of mermaids in this plodding stretch of film, but it's very brief.
