Here is one of my favourite stories of mermaids. I'll put in three different versions of it. Which one you like best, decide for yourself. Enjoy reading!
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1. The Crofter and the Seal-Woman
There once was a young crofter (a scottish farmer),
who owned a little patch of land on the coast. One
day while walking by the beach, he saw a group of
beautiful women dancing in the sunset, he saw a
pile of seal skins beside them and knew they were
selkies. As the sun went down they put on their
skins and slid back into the water, but as the last
selkie reached for her skin, the crofter jumped out
and seized the skin from her. She begged him for
it, but he wouldn’t give it to her and in the end she agreed to be his wife.
She was a good wife and gave him 7
beautiful children, but she yearned to
return to the sea. One day her
youngest child showed her a box he
had found in the barn, in it was her
seal skin. When the crofter returned
from the fields, his wife was gone,
back to the sea.
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2. The Mermaid Wife (Shetland Islands)
A story is told of an inhabitant of Unst, who, in walking on the sandy margin of a voe, saw a number of mermen and mermaids dancing by moonlight, and several sealskins strewed beside them on the ground. At his approach they immediately fled to secure their garbs, and, taking upon themselves the form of seals, plunged immediately into the sea. But as the Shetlander perceived that one skin lay close to his feet, he snatched it up, bore it swiftly away, and placed it in concealment.
On returning to the shore he met the fairest damsel that was ever gazed upon by mortal eyes, lamenting the robbery, by which she had become an exile from her submarine friends, and a tenant of the upper world. Vainly she implored the restitution of her property. The man had drunk deeply of love, and was inexorable; but he offered her protection beneath his roof as his betrothed spouse. The merlady, perceiving that she must become an inhabitant of the earth, found that she could not do better than accept of the offer.
This strange attachment subsisted for many years, and the couple had several children. The Shetlander's love for his merwife was unbounded, but his affection was coldly returned. The lady would often steal alone to the desert strand, and, on a signal being given, a large seal would make his appearance, with whom she would hold, in an unknown tongue, an anxious conference.
Years had thus glided away, when it happened that one of the children, in the course of his play, found concealed beneath a stack of corn a seal's skin; and, delighted with the prize, he ran with it to his mother. Her eyes glistened with rapture -- she gazed upon it as her own -- as the means by which she could pass through the ocean that led to her native home. She burst forth into an ecstasy of joy, which was only moderated when she beheld her children, whom she was now about to leave; and, after hastily embracing them, she fled with all speed towards the seaside.
The husband immediately returned, learned the discovery that had taken place, ran to overtake his wife, but only arrived in time to see her transformation of shape completed -- to see her, in the form of a seal, bound from the ledge of a rock into the sea. The large animal of the same kind with whom she had held a secret converse soon appeared, and evidently congratulated her, in the most tender manner, on her escape. But before she dived to unknown depths, she cast a parting glance at the wretched Shetlander, whose despairing looks excited in her breast a few transient feelings of commiseration.
"Farewell!" said she to him "and may all good attend you. I loved you very well when I resided upon earth, but I always loved my first husband much better."
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3. The Silkie Wife (Shetland and Orkney Islands)
Those in the Shetland and Orkney Islands who know no better, are persuaded that the seals, or silkies, as they call them, can doff their coverings at times, and disport themselves as men and women.
A fisher once turning a ridge of rock, discovered a beautiful bit of green turf adjoining the shingle, sheltered by rocks on the landward side, and over this turf and shingle two beautiful women chasing each other. Just at the man's feet lay two sealskins, one of which he took up to examine it. The women, catching sight of him, screamed out, and ran to get possession of the skins. One seized the article on the ground, donned it in a thrice, and plunged into the sea; the other wrung her hands, cried, and begged the fisher to restore her property; but he wanted a wife, and would not throw away the chance. He wooed her so earnestly and lovingly, that she put on some woman's clothing which he brought her from his cottage, followed him home, and became his wife.
Some years later, when their home was enlivened by the presence of two children, the husband, awakening one night, heard voices in conversation from the kitchen. Stealing softly to the room door, he heard his wife talking in a low tone with someone outside the window. The interview was just at an end, and he had only time to ensconce himself in bed, when his wife was stealing across the room. He was greatly disturbed, but determined to do or say nothing till he should acquire further knowledge.
Next evening, as he was returning home by the strand, he spied a male and female phoca sprawling on a rock a few yards out at sea.
The rougher animal, raising himself on his tail and fins, thus addressed the astonished man in the dialect spoken in these islands, "You deprived me of her whom I was to make my companion; and it was only yesternight that I discovered her outer garment, the loss of which obliged her to be your wife. I bear no malice, as you were kind to her in your own fashion; besides, my heart is too full of joy to hold any malice. Look on your wife for the last time."
The other seal glanced at him with all the shyness and sorrow she could force into her now uncouth features; but when the bereaved husband rushed toward the rock to secure his lost treasure, she and her companion were in the water on the other side of it in a moment, and the poor fisherman was obliged to return sadly to his motherless children and desolate home.
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Link to a full version:
http://www.geocities.com/RainForest/1620/merwife2.html