Seer of Sevenwaters by Juliet Marillier — book slip

Sibeal, a young druid seer, is on the verge of taking her final vows when she is sent to spend the summer on Inis Eala, an island known for its elite warriors.

During her stay, a Viking shipwreck brings three survivors to the island: Knut, a friendly Norseman; Svala, his mute and enigmatic wife; and Felix, a young scholar suffering from amnesia.

As Sibeal helps nurse Felix back to health, they form a deep bond, leading her to question her commitment to the druidic path. A runic divination reveals that Felix must embark on a perilous mission, and Sibeal is destined to join him.

Together, they face treacherous waters and mythical sea creatures, challenging both their physical endurance and inner convictions.