The cunning man - Celia Rees

Finn is haunted by terrifying dreams where a huge wave threatens
to engulf her in watery darkness. They have been worse since she
arrived at the Cornish cottage.
Then she discovers the strange curse that was laid upon the cottage
years ago, and she's sure vengeful Griffiths has something to
do with it. She's seen him, endlessly tying and retying a length
of rope, creating the knots that - superstition has it - can conjure
storms. Finn knows that she's in mortal danger if she can't find
a way to break the curse...
A superbly spine-chilling teen thriller from the award-winning author
of WITCH CHILD