well, there's two different sorts of creatures that are similar to mermaids. I don't know about true mermaids, but I know a bit about sirens and selkies.
sirens, from greek myth, have been described both as pretty women with fish tails (so essentially mermaids), and ugly bird-like hags with nice singing voices (so essentially harpies). despite the contradictions on what they look like, it's generally agreed that they have beautiful singing voices which they use to maliciously lure sailors to sharp rocks and basically drown them. if you hear their song, you become obsessed with finding the source of the voices, until you reach the island and either drown, or get eaten by the sirens.
one famous hero (I forget the name) had plugged the ears of everyone on the crew, except him, because he wanted to hear the voices. the hero had himself tied up so he didn't jump into the water and told everyone to ignore his orders until the voices stopped. it worked. Pretty much the only way to hear a siren's voice and live.
Selkies are from irish myth (I think), and I don't know as much about them as sirens. what I do know is that they have the lower half of a seal, and have the ability to turn into a human. when they shapeshift, they leave behind a seal coat, which they must be wearing in order to regain their true form. one can steal this coat and leave the selkie landlocked indefinitely.
when a selkie falls in love with a human... something happens. I forget. someone google selkies!
I know nothing about true mermaids, other then that they appear to be a more sympathetic version of sirens. :/