I found this story of a girl marrying a water snake in a large book of Russian myths.
In the depths of a lake, a human girl lived with her snake husband, who in the water took the form of a man, for many years. One day she 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.'"During the visit, her mother asked how she was to get back to her husband. The daughter explained little knowing her mother's plan.
That night, her mother when to the lake with an ax and 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.
