On previous posts I have discussed about the celtic belief in lakes, rivers, and wells, believes later christianized and concealed under the characters of Saints deeds and sancutaries .I would like to share with you some quotations regarding water maidens from the book "British Goblins - Welsh Folk-lore, Fairy Mythology, Legends and Traditions by Wirt Sikes -  " (Obtain this full work and many more backups by clicking here!)
In all these legends the student of comparative folk-lore traces the ancient mythology, however overlain with later details. The water-maidens of every land doubtless originally were the floating clouds of the sky, or the mists of the mountain. From this have come certain fair and fanciful creations with which Indo-European folk-lore teems, the most familiar of which are Undine, Melusina, Nausicaa, and the classic Muse. In Wales, as in other lands, the myth has many forms. The dispersion of dark clouds from the mountains, by the beams of the rising sun, or the morning breezes, is localized in tile legend of the Men of Ardudwy. These men make a raid on the maidens of the Vale of Clwyd, and are pursued and slaughtered by the latter's fathers and brothers. The maidens thereupon cast themselves headlong into the lake, which is thenceforth called the Maidens Lake, or Llyn y Morwynion. In another legend, the river mist over the Cynwal is the spirit of a traitress who perished long ago in the lake. She had conspired with the sea-born pirates of the North (the ocean storms) to rob her Cambrian lord of his domains. She was defeated by the aid of a powerful enchanter (the sun), and fled up the river to the lake, accompanied by her maidens, who were drowned with her there. ['Arch. Camb.,' 4th Se., vii., 251]
As the mermaid superstition is seemingly absent in Wales, so there are no fairy tales of maidens who lure mortals to their doom beneath the water, as the Dracae did women and children, and as the Nymph of the Lurley did marriageable young men. But it is believed that there are several old Welsh families who are the descendants of the Gwragedd Annwn, as in the case of the Meddygon Myddfai. The familiar Welsh name of Morgan is sometimes thought to signify, 'Born of the Sea.' Certainly môr in Welsh means sea, and gân a birth. It is curious, too, that a mermaid is called in Basse Bretagne 'Mary Morgan.' But the class of stories in which a mortal marries a water-maiden is large, and while the local details smack of the soil, the general idea is so like in lands far remote from each other as to indicate a common origin in pre-historic times. In Wales, where the mountain lakes are numerous, gloomy, lonely, and yet lovely; where many of them, too, show traces of having been inhabited in ancient times by a race of lake-dwellers, whose pile-supported villages vanished ages ago; and where bread and cheese are as classic as beer and candles, these particulars are localized in the legend. In the Faro Islands, where the seal is a familiar yet ever-mysterious object, with its human-like eyes, and glossy skin, the wife of supernatural race is a transformed seal. She comes ashore every ninth night, sheds her skin, leaves it on the shore, and dances with her fairy companions. A mortal steals her sealskin dress, and when day breaks, and her companions return to their abode in the sea, compels her to remain and be his wife. Some day he offends her; she recovers her skin and plunges into the sea. In China, the superstition appears in a Lew-chewan legend mentioned by Dr. Dennys, ['Folk-Lore of China,' 99] which relates how a fairy in the guise of a beautiful woman is found bathing in a man's well. He persuades her to marry him, and she remains with him for nine years, at the end of which time, despite the affection she has for their two children, she 'glides upwards into a cloud' and disappears.