Finfolk were magical creatures of the sea closely related to the Selkie, in the belief of early islanders the Selkie was a type of mermaid that resembled a seal but was relatively harmless if left alone - in stark contrast the Finfolk were prone to piracy, raiding, shipwrecking, trickery and most terrifying of all human abduction.
Indeed for many years islanders lived in fear of Finfolk, who were said to prefer human spouses over marriage to their own kind - however this spouse didn't have to be a willing one and often they would snatch unwilling mortals to accompany them and even though their human victims often struggled and cried for help the Finfolk were uncaring.
Finfolk males were highly aggressive and shunned human company except when they entered "his" domain - which was often wherever their was good fishing ground - the islanders, who depended heavily on fishing to survive, often came into conflict with territorial Finfolk males who would stir up the waves to unbalance boats, cut fishing line, ram into boats to cause holes in the hull, snap off oars or rudders and generally cause havoc.
The best defence against these aggressive Finfolk was said to be the practice of drawing a white cross on the underside of a boat, Christian imagery was disliked by many magical beings and the Finfolk absolutely loathed it - the mere sight of it would cause them to swim far away and leave their would-be-victims in peace.
When not terrorising fishermen or sailors the Finfolk males would often surface and take the form of fishermen themselves, sailing the sea of small boats - they were said to be supernatural experts at rowing, capable of swimming over the islands in a few strokes: many times when they took this form the Finfolk males were "fishing" for their own particular prey, human females - if they found any they would snatch them away to be unwilling brides: being on land was not always enough to protect oneself as Finfolk males could easily steer their boats onto the shore and walk on land, though they didn't venture too far inland so in theory they could be outran but this was likely a dangerous thing to try and Finfolk males were aggressive enough at the best of times.
As for the Finfolk females, they were said to be a lot less aggressive than their male counterparts and started life as generally attractive mermaid-like creatures, however if they were to marry a male of their own species they were doomed to progressively grow uglier with age until they became a hag - also marriage to a Finfolk male was often a cruel one in which he may beat his spouse so harshly she would be bed-ridden for days.
Perhaps somewhat understandly Finfolk females would much rather have a human spouse but much like the Finfolk males they had aggressive tactics when it came to courting - although they rarely chased their prey they would often trick or seduce them before dragging them away: presumably Finfolk females were incredibly strong as once she latched onto her victim they had little chance of escape.
Once captured by a Finfolk the human victim was taken to live in their underwater kingdom where they would be forced into the servitude of their wife or husband - doing all the tasks expected of them, presumably with severe consequences if they did not: it was not uncommon for Finfolk to treat their stolen spouses like slaves and although less violent in nature it was unlikely Finfolk females were any less cruel towards their spouse in regards to forced labor.
Although generally malignant in nature Finfolk were said to occassionally have access to a secondary home in the form an island that only appeared at certain times of the year, this island was said to be akin to a paradise and the Finfolk would often take their stolen spouses with them to this island, perhaps the one time of the year that their human "companions" could have some rest from their usual tasks.
'Fin' comes from 'Finnar', the non-norse inhabitants of Scandinavia now known as the Saami, though probably also including a related group of people who still use the name - the Finns. The Norse settlers of Orkney brought over their legends about the Finnar, with whom they had frequently interacted and intermarried in Scandinavia, and who were already known and employed as both sorcerors and expert boat builders. The Finnar were known to build light and fast boats that could literally not be caught by larger heavier Norse boats, and they were also reputed to have magical control of wind and wave. Because of these skills, they built and often manned many Viking boats, crossing into many Viking colonies as sailors or slaves. Their spiritual practitioners were also rumored to be shape shifters. This cultural contact informed the stories of the finfolk, who were more memory than fairytale, and laid the foundation for the post-Viking shift away from Scandinavian lore. Exposure to English in the Orkneys led to the understandable confusion about these magic boatmen having actual 'fins', increasing their association with the aquatic. This in turn smoothed the transition into a more homegrown aquatic shapeshifting form - the seal. Once associate with the seal, the tales spread farther south along the Celtic coasts where seals were a common element of local lore, well past the line of Norse settlement where memory of the Finnar ended. (See: Orkneyjar.com - The Root of the Finfolk Myth)