I found this story of a girl marrying a water snake in a large book of Russian myths.
One day a young girl was swimming in a lake with a bunch of her friends. When they were tired of their fun they came out of the lake and put their clothes back on, except for the one girl who found a snake laying on her outfit. She begged him to go but instead he bargained with her. He said that if she agreed to marry him she could have her clothes back. Her friends told her to go ahead, after all who could marry a snake? So she accepted the snake's proposal and he left.
Later, the girl told her mother what had happened. “Do not worry”, her mother said, “who can marry a snake?” Several days later, a large group of snakes appeared and forced their way into the house. They took the young girl and dragged her back to the lake. Once underwater the snakes turned into people, and the girl was married to the snake.
After some years, the girl asked to visit her mother and her husband agreed. He took her and their children to the surface and said, "When you wish to return, come to the lake and call 'Joseph, Joseph, come and fetch me.'"
The girl brought her children to her mother's house and had a friendly visit. She told her mother that her life in the lake was very pleasant, better even than her mother's. During the visit, her mother asked how she was to get back to her husband. The daughter explained little knowing what her mother was planning.
That night while the girl slept, her mother sharpened an ax and went to the lake. Once there she called, "'Joseph, Joseph, come and fetch me." When the snake appeared she cut off his head. Of course, she did not tell her daughter about this.
The next day, when the girl returned to the lake and called, her husband did not appear. After a time she found a head floating in the water and realized what had happened.
"My mother has killed him," she said.
Using the magic she had learned from the snake people, she turned herself and her children into birds.