Ann Blyth is absolutely gorgeous (her beauty is the highlight of the film), but she has nothing to work with. She never says a word and spends the entire film simply floating around in the water. Also the character of Powell's wife (played by Irene Hervey) was completely unlikable. Slow pace, non-existent humor (example: the mermaid cries while underwater and wipes her tears away with a handkerchief. Hardy-har-har.), woman littering on the beach, very little mermaid swimming action, dead on arrival acting, unsatisfactory ending.
I love William Powell, but this movie is lame as Hell. Skip it.
It's a little known fact that the girl that Frankenstein's monster threw into the lake actually turned into a mermaid and that the monster himself escaped the fire at the mill. They later hooked up and ran off to Hollywood, CA were they lived happily ever after and operated a pet store for the next 54 years until it burned down. The fire was captured in the documentary PEE-WEE'S BIG ADVENTURE. After collecting the insurance money from the fire they retired and moved to Cedar Park, TX where I still occasionally see them holding hands.