Splash

Not only is it funny and the plot wonderful, but there is quite a bit of awesome footage of Daryl Hannah mermaiding in the ocean.
We can't get enough of that! Keep in mind that this is not a children's movie. It was made in the '80s, and for the most part it is okay for the family to watch. But there is a little bit of nudity (when the mermaid is walking into New York out of the ocean with nothing but her long hair to cover her) and there is some implication of what she and the Tom Hanks character are doing in the elevator, though nothing is shown. The movie is wonderful, with a great story and awesome mermaid footage and lots of laughs, as well as a great romance.