WHAT IS IT?
A selkie is a mythological  creature who lives as a seal in the sea but shed their skins on the land and become human. Almost all of the selkie stories are romantic tragedies  Sometimes the human doesn't know of their lovers dual nature and is mystified when they just vanish into the night.
Other tales have the human hiding the Selkies seal skin which prevents it from changing back. There are many variations on this theme.
WHERE DO YOU GET IT?
Back to the oceans of the world we go. This time we'll lie in wait around the waters of Scotland and Ireland  Since we'll be moving around the seal colonies it's going to be cold, very wet and incredibly smelly.
But faint heart ne'er won true lady right? right
USING IT IN REAL LIFE:
The most obvious use for a Selkie is to hide her skin, marry her and then do nothing until she manages to escape back to the sea.
Whereupon you notify the police of your wife being missing, undergo a three ring media circus with clowns AND a dancing bear.
Get accused of doing her in, be found not guilty due to lack of evidence and a very flimsy case.
Finally collect on the massive amount of life insurance that you took out on her and live it up.
USING IT IN WRITING
Selkies are often painted as victim le tragique in the stories but what if they weren't? Perhaps there have been a rash of attacks in the town and nobody can figure out why there is a particular odour at the scene of the crime 
A group of Selkies might run something of a protection racket on the fishermen. If they don't get their cut then the fish are all driven away from their boats.
It might be something to play around with in the background of a character. Perhaps he's going through the change for the first time and freaking out about it 
Does your Selkie have access to Selkie Magic of some kind? Will he use his powers to protect or harm?
MIGHT ONE TAKE OVER THE WORLD WITH IT?