I hope this is an okay topic ! It's one that I find very interesting. The Rusalki are similar to fresh water sirens. They're not as well known though.
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According to Slavic folklore, a Rusalka is a water spirit or water nymph. They are the souls of young women or girls who died an unnatural or violent death. If the girl was murdered in or close to a lake, she would became a Rusalka and inhabit that particular lake. Rusalkas appear as beautiful young women who try to lure men into the water, where they will drown them. Rusalki can also be found on nights when there is a new moon; dancing on meadows or open places in the woods. With their shrill laughter they are capable of killing humans. A Rusalka's fate can be undone by avenging her death.
Whenever people would bathe in the lake they put fern in their hair so that the rusalka would not pull them under and drown them.
www.pantheon.org/articles/r/rusalka.html
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Rusalka in Russian folklore appears as a beautiful maiden or assumes the form of a siren: a halfwomen and half-bird or half-fish. She is a spinner who regulates human and animal fertility, the cycle of the seasons, the moon and the weather. In folk tales, she is one of the many daughters of a sea or birdking, and her mysterious underworld nature is feared. Wherever she reveals herself, in forest groves or by the water, she is not to be angered or disturbed. Sometimes she is a water nymph who drowns all who venture into her territory, or she sings in order to lure men and destroy them. There was widespread belief among the Russian peasantry that rusalki were the souls of girls who had drowned themselves because of desertion by a lover or rejection by a parent.
tspace.library.utoronto.ca/citd....html
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Russian Myths: The Rusalka
The rusalka of Russian myths are the spirits of young women who were murdered before marriage and are then cursed to live in a lake in the form of a mermaid. There they will sing sweet songs to entrap men into the water and drown them. A rusalka can be released from her demonic form if someone avenges her murder.
The rusalka are slim with long, loose hair, blazing eyes and magnificent breasts. Their hair may be light brown, blond or green. They can assume the form of a fish or have legs like a human. In the latter form, they haunt the forests, dance with the moon and swing from the branches of trees. Often we see them sitting on the bank laughing with their friends the water sprites. Sometimes they visit local villages to join in the dances and entice men into their lakes to become their husbands or kill them.
If you would like to go swimming with the rusalki (plural of rusalka) put fern in your hair so they cannot pull you under and drown you. Some say that only witches can swim safely with rusalki.
Another group of Russian myths claim that the rusalka are water nymphs who marry the Wodjanoj. The Wodjanoj are male water spirits who live in great castles under the water and can change their shape at will.
Marriage alters the rusalka. She goes from wild and lustful to sweet and demure.
I see many obvious links between the rusalki of the Russian myths and the mermaids of Celtic myth. Both are beautiful, sexually liberated and occasionally dangerous. They are both descended from goddesses of fertility and retain some of their characteristics. One article I read makes especial reference to the hair of the rusalka. It is loose and uncontrolled like the rusalka themselves. Notice how even now we associate loose, wild hair with sexuality.
www.beautiful-mermaid-art.com/rus...html
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Also see:
en.wikipedia.org/wiki/Rusalka
"The Rusalki: Slavic Nature Nymphs" --- www.mythicarts.com/writing/Rusalki.htm
library.thinkquest.org/C00832...lka.htm
Some mentions here: www.mythinglinks.org/euro~ea...vic.html
www.monstropedia.org/index.php
This has other water folk as well: www.beautiful-mermaid-art.com/mer...html
Mentioned: www.creativeworkfund.org/pages...g.html
www.stonedragonpress.com/wicca...a.html (nice little outline)

I just wanted to share! Out of all the water lore, they're my favorite.