Though it would be fun to try and put together a helpful list of mermaid movies/TV shows that people have seen or heard about. Either that is focused about mermaids or where they appear along the way.

So far I've got:

The Little Mermaid (1989)
Splash (1984)
Mermaids (2003. A TV film about three mermaids who are sisters.)
Pirates of the Caribbean 4: Stranger Tides (2011)