To be honest, femme fatale wasn't the first thing that came to mind. (or the second, really) Tail is kinda scary, reminds me a bit of seal tails. At the end of the tail, is that a barb? As for the arm...? I don't see a problem with it. I do like the braided hair.
Question: if she's part of an underwater society with natural predators, wouldn't having hair that long be a no-no? It's really pretty and all, but I'd think that it would get in the way, snag on coral, impair vision and the like. Wouldn't they either not grow it that long (biologically) or try to cut it short (with a knife)?