In Slavic mythology evil (male) spirits usually live in water nearby watermills.
Those spirits, discovering from Serbian stories, used to be humans that usually did evil to other humans. Their punishment is to live forever in water. People believe that damage on watermills and bridges is made by those evil spirits.
Vodenjaci (that’s how people in Serbia call them) make people drown and become their slaves. To stop them from doing that evil, people usually sacrificed chicken or a sheep. (via Radonich Aleksandra)