The Sea Witch: A Little Mermaid Retelling by Rebecca F. Kenney

When Averil saves an attractive prince from a shipwreck, she thinks she feels the Soul Echo, the sign that the young man is her one true mate. She's desperate to gain human form and walk on land... and the malevolent Sea Witch is all too pleased to offer her a deal that could make all her dreams come true. He encourages her to win the prince's heart, and he even coaches her in the art of seduction. But what is the real price of the Sea Witch's help? And why, despite his grouchiness, is he so much more handsome and charming than Averil expected?