The Sea Witch by Rebecca F. Kenney

When Averil saves a young man from a shipwreck, she thinks she feels the Soul Echo–the sign that the young human male is her one true mate. And the malevolent Sea Witch is all too happy to offer a deal that could make all her dreams come true. He wants her to win the prince’s heart–even coaches her in the art of seduction. But what is the real price of the Sea Witch’s help? Why is he more brutally beautiful–and charming–than Averil expected?