Mermaids are beautiful creatures on the outside. And while they can be just as beautiful on the inside, for the most part they aren't. They use their beauty and voice to lure their victims into the water so that they can drown them. (Very charming, I know). Pirates of the Caribbean: On Stranger Tides (movie 4) is a wonderful example of the true nature of Mermaids.
Merman are very different on the other hand. They hardly ever come to shore and prefer to go unseen by human eyes.