In 1653, Swedish witch Karin Persdotter confessed to having learned her magic from a male water spirit, called variously the "man of the stream" (strömkarlen), "the river" (älven), and the "nix" (näcken) (Hall 32).
Readers of the Brothers Grimm will recognize this latter term: the nix (masculine) and nixie (feminine) (German nix and nixe) have haunted the rivers, lakes, and ponds of folk tales for (apparently) several millennia at least. They are, in effect, fresh water merfolk.
The Hwicce, the Anglo-Saxon tribe ancestral (some say) to today's witches knew a similar species. Their nicor survived in English folklore as the nicker or knucker. The youthful Beowulf was said to have wrestled with several while swimming.
In fact, all these names descend from the same ancestor: proto-Germanic *nikwiz, *nikwuz (Watkins 59). To judge by surviving folklore, all the Indo-European-speaking peoples knew of the People of the Waters. But of course, other peoples know them too; everyone knows them. Here in Minnesota, the Anishinabe (Ojibway) call them nebaunaubaequaewuk.