I've read about rusalkas before now. Some stories paint her as the ghost of a girl who had committed suicide by drowning or had been murdered. Her restless and unhappy spirit turns her into a vampiric creature that gains a kind of fake life for herself by absorbing life from everything around her. One of the so-called signs of a rusalka is that everything around the water where she died is dead. There are no living plants, trees, animals or even moss. This gives her enough life to leave her watery habitat in a limited radius. If she encounters a human being (usually male), she enchants him and slowly leeches away his life essence. At this point, she gets strong enough to travel miles away from the place where she died and seek out other victims instead of waiting for them to stumble upon her. Villagers who know this post signs warning travelers away from her spot and, without human sustenance, her life force wanes to the point where she is trapped in her watery grave. She never dies completely (because she's already dead) but she can be neutralized and rendered harmless to human life with the proper precautions. I really like your rendition here, one that succinctly captures the deceptive nature of this dangerous water demon.