This is the first of three entries based off of Pirates of the Carribean: On Stranger Tides. One of the subplots in that movie involves a mermaid. I'm also going to be drawing in some inspiration from another movie about a mermaid . . . no, not The Little Mermaid (that took place in the same universe as the other animated princess movies), but Splash. (Splash was made by Touchstone Pictures and distributed by Buena Vista, which means it's at least connected to Disney).
Mermaids as presented in On Stranger Tides are basically predators. They take the form of beautiful women, from the waist up, and use their beauty and their singing to draw sailors close enough to, well, eat. However, rather than killing a man, they can choose to give him the ability to breathe underwater. Likely, this is how they breed, as the pirates agree that mermaids have no men among them (presumably they cannot bear male children). When removed from the water, the fishlike tails of mermaids will separate into legs, making them indistinguishable from human women, at least until they get wet again.
In Splash, there's no indication that mermaids eat men. However, we do see Madison, the mermaid who comes on land to pursue her love for Tom Hanks, eat a lobster by tearing through its shell with her teeth. As in On Stranger Tides, we have mermaids who turn human when out of water and who can give men the ability to breathe underwater. Splash adds some extra detail to the mermaids, giving them their own language (which resembles the sounds of dolphins), the ability to learn a language in an afternoon, a city under the sea, and the idea that there is a time limit on her visit to the surface world. As I interpret it, if Madison stayed on the surface until the full moon, she would never be able to return (the line in the film was a little vague, and apparently a deleted scene referred to a bargain struck with a witch, much like The Little Mermaid). Men who are taken under the sea by mermaids cannot return to the surface (presumably losing the ability to breathe air), but they retain their human form.
Fitting it into the WW4C
Until now, the WW4C doesn't have an Atlantis, but the city under the sea that we see at the end of Splash can serve. Since this city was built by the mermaids, it doesn't fit perfectly as it was never sunk, but if you need heroes who come from an undersea kingdom, this fills the role.
At the time of On Stranger Tides, mermaids are common knowledge, and although they will occasionally take men to live with them, their primary use for humans is as a food source. However, one mermaid, known as Syrena, falls in love with a missionary and takes him to live with her.
Presumably, the missionary teaches the mermaids that they should not prey on mankind, and the mermaids hide from humans. By the mid-1980s, they are only known as myths until the mermaid called Madison is exposed and briefly becomes national news. Mermaids are known to exist now, although the location of their city is unknown.
I'm going to posit the existence of good mermaids, who live in an undersea city, and take men to husband voluntarily (or mate on the surface and then return below), and evil mermaids, who did not abandon the old ways and still attack and kill sailors.
Heroes could include mermaids or men who live among the mermaids (and require breathing apparatus on land but can survive in the ocean depths unaided). Villainous man-eating mermaids could serve as a threat to heroes in general, or as a barbarian horde devoted to the destruction of the kingdom of the mermaids.
