The most popular image of the mermaid is a half-girl, half-fish beauty. Biologically, physiologically and evolutionarily, this sort of mixing is 0% compatible.
I'm also not going for the half-girl, half-whale alternative because I have a feeling that it's been done several times before.
No, this kind of mermaid is half-girl, half-sea-lion. Let's assume that such a human-sea-lion half-and-half exists. Physically, biologically and behaviorally, where would the human end make way for the sea lion end?