Lady of Weeds - Gingell W.R.

Beware the gift left too close to the sea...

Cold, isolated, and almost a thing of the sea herself, Carys keeps the seashore. It's a simple calling: collect the seaweed along the rocky shore every day before the selkies come out to play, or pay the consequences. She knows the workings of the laws between the sea and the shore, and she collects payment from the sea itself. Issues of King and country barely touch that moving, foamy border: only saltwater, selkies, and seaweed exist by the rocky shore. Every day is the same for Carys; a cold continuity of service.

Then a boy washes up in one of Cary's tide-pools: injured, amnesiac, and bearing with him a token of her past without the memories to accompany it. Now she will need to keep him safe from enemies both at land and at sea; but the land is a mystery to her, and as for the sea—no one knows better than Carys that a single slip of the foot means death by the rocky shore.

Or that for every gift given by the selkies, there must be payment in one form or another...