Well, I figure that most mermaids (bar the bathypelagic ones) live close to the shore (in the reefs) and go on land regularly. So they'd need reasonably good eyesite for being on land and hearing and breathing apparatus that can work in both environments. Plus I'm willing to cheat just a tiny weeny bit on the mermaid's appearance (well, some of the races) because, really, who wants to read about an ugly mermaid? :-P

That thing about gestures and body attitude is interesting. It'll make things interesting for Miriam when she's in Amaryllis' body!