I'm sorry I neglected to put up a little background on Selkies on my previous Flash Fiction.
I'll just borrow from Benjamin Russell's comment: "Selkies are able to become human by taking off their seal skins, and can return to seal form by putting it back on. Stories concerning selkies are generally romantic tragedies. Sometimes the human will not know that their lover is a selkie, and wakes to find them gone."
I'll also add that the lore can be found in Ireland, Scotland, Iceland, and other islands nearby. It is said to have originated in Scotland: Orkney and Shetland Isles. The word "selkie" means seal in the dialect of the Orkney Isles. Nowadays, they are often lumped together with Fairy races.
Selkie-men were notorious for seducing women (unwed and wed) and luring them into the sea to live with them. This was a common (and handy) explanation for when a village girl went missing.
These are the typical scenarios that occur from a selkie encounter...but not always!
I have a passion and fascination for mythology and folklore, mythic creatures, but especially those who can manipulate form and become something else: shapeshifters, shape-changers, skinwalkers, were-creatures, etc...
I have some wonderful resources on them, including metaphysical and spiritual sources and have even had personal experience with shifter energy. This energy was the inspiration for my own romantica story, "Shifting Illuminations."
When I am more clear-headed, I will share some of that story. I am currently working on editing it to submit to a publisher. I will also share my own take on a shapeshifter myth/fairytale that I wrote in college for a writing assignment. I chose the Swan Maiden tale and gave it an elemental spin.
Selkie on-line Sources: