The only thing they have in common is the use of selkie mythology. Both were good in their own way, but I'd say each film took very different directions, I don't see the resemblance at all.
No 21st. As the other person replied, other than "Sallie" mtthology, Jordan wrote this story, as he almost always does. There are a couple of exceptions, and I think he was out If his element one was his previous film "The Brave One", starting Jodie Foster, a brilliant actor. The film has some very good moments, but I don't find it as near as good as the Ondin e and some of his other work. I thought it was interesting that originally Nation Cotillard was going to play the part of Ondine, but she had a prior commitment. She is a truly gifted actor; that said, I think Ms. Bachleda did a magnificent job in her role. Plus, look what happened between her and Farrell!
Please forgive the grammatical errors. It is due to the broken screen and autspell on my cheap phone. I obviously meant "Selkie" and "Marion". ...Plus other errors you can work out.