Oh yes, that rings a bell, I have some books on scottish and Irish tales, will look the Silkie Wife up. Obviously, Russian folk culture isn't so sea-based, but there are the Rusalki, who are pretty much mermaids, but they seem to inhabit lakes, pools, and rivers. They are said to be the spirits of maids who have drowned. They try to entice mortals to their death, to join them. So, these are human in origin, but there is also the vodianoi (voda in Russian means water). He is also seen as malevolent, but fishermen may win his good favour by giving gifts, and in fact, when I went out on lake Baikal, Siberia, the captain threw a tot of vodka over the side, so this tradition is still going.