The Woman from the Waves by Roslyn Sinclair

Six years after a mysterious woman saved her from drowning off a remote Scottish island, Madeleine Laurent, who has since left her convent, crosses the ocean to find her rescuer. Instead she meets Hæra North, a brazen local who stirs her repressed desires. Hæra is an ocean horse spirit who hopes to become the clan's first female Stormhorse by capturing, drowning and eating a worthy human. She chose Madeleine, then saved her instead, and now plans to win her trust and devour her, until Madeleine awakens a different kind of hunger. A sapphic romantasy.