Other!
Vampiric or neo-vampiric, supernatural creatures who dwell under water, there is, for one example, the water-horse.
The Celtic water-horse is most well known today as the kelpie. Variations of the kelpie can be found in the lore of Ireland, the Shetland Islands, the Isle of Man, the Scottish Lowlands and the Scottish Highlands.
Considering both shape-shifting ability and voraciousness, the kelpie of the Scottish Highland and the Island of Man in the Irish Sea was the most fearsome.
One of its tricks was to take the form of a beautiful horse and lure children onto its back. It then headed for the loch. After jumping into the loch with its victims, it devoured all of the flesh and blood within their bodies except for internal organs such as livers, hearts, and lungs, which then came floating to shore.
In human form, the Highlands kelpie sometimes behaved like a a true vampire. In one story, a kelpie took the form of an old woman and begged some girls tending cattle to share their shelter with her. The girls consented. One of the girls woke up in the middle of the night and saw the old lady sucking blood from one of the other girls. The girl managed to escape and tell the tale.
The Highlands kelpie also sometimes took the form of a handsome man to take advantage of women. Typically, he had trouble maintaining human form. In one tale of a woman being courted by a kelpie, the masquerade is foiled when the woman notices that the 'man' has horse-hoofs. In another, the man's hair begins to turn to sea weed.
The folk beliefs found on the Orkney Islands off the coast of Scotland include both the kelpie and the 'nucklelavee'. The nuckelavee somewhat resembles a centaur. The upper body is basically humanoid and the lower body is basically equine. But it had flippers instead of hooves, and it's head, like that of a cyclops, had only one eye. It had neither skin nor scales – the whole surface of its body displayed naked flesh with yellow veins through which blood pulsed like black tar. It would come to land to prey upon people and domestic animals. Also, its foul breath could blight the crops. It could not dwell in fresh water or cross over fresh water, and in the folk tales about it this often provided a way for a person to escape the creature's pursuit.
Also to be considered are the 'worms', the dragon-serpents of Anglo-Saxon, and Scottish lore. Some of these lived in the sea and can be considered to be sea serpents. But even the land versions often dwelled in wells and lochs. Both preyed upon humans and livestock, and their fowl breath could blight crops and cause epidemics of disease among humans.
So far I haven't found any clear-cut cases in myth, legend, or folktale where a mermaid or neo-mermaid was an outright blood-sucker. But it doesn't seem like too much of a leap to write a story about such a vamp. After all, if the 19'th century creators of the modern literary vampire such as Bram Stoker had got hung up in making their vampires exactly match those revenants in folklore which are their basis, they would not have succeeded in their efforts.
Bram Stoker indeed did write a story which involves both Medieval legends about the "worm" and the image of the lamia as a vampish snake-woman: The Lair of the White Worm. In this story, a gigantic, blood thirsty "Worm" dwells in a deep well on an estate in England whose history goes back to at least the time of the Roman occupation.
One of the story's protagonists, the uncle of the story's central hero, gives a neo-Darwinian explanation for the existence of such a creature and its presence in old legends and in their neighbour's well. In prehistoric times such enormous serpent-like creatures lived in the great swamps and marshes that surrounded the mouths of European rivers. As the climate changed, they adopted to dwelling in natural caverns and tunnels that extended from the bed of these wetlands, and as part of this they also developed the ability to tunnel through clay and other soils. At the same time, their mental powers evolved greatly.
The snake-woman in the story, Lady Arabella, is the widow who now owns the estate and its well. She was once a nice young girl but one day while out in the countryside alone, she received a venomous bite. After she was found and brought home, she was at first severely ill. Then, suddenly, she suddenly had a remarkable recovery. But she was no longer a nice girl. The central hero's uncle deduces that she actually did die from the bite, and the White Worm took possession of her as soon as her own soul departed from her body.
The creature is still dependent on dwelling in the well in worm-form part of the time, but, in the form of Lady Arabella, it has taken full control of the estate.
There is one passage near the end of the story that is quite erotic in a vampish way:
"She [Lady Arabella] tore off her clothes with feverish fingers, and in full enjoyment of her natural freedom stretched her slim figure in animal delight. Then she lay down on the sofa – to await her victim! Edward Caswall's life blood would more than satisfy her for some time to come."
There is a movie spin-off of Stoker's story available as a VCR tape at video stores which is well worth watching: The Lair of the White Worm, produced in Great Britain in 1988, starring Ken Russell and Amanda Donahoe. There are many differences between this movie and the original story. It's really a whole new story. But it preserves and enhances the best elements of the original story. It's actually better than the original.