Water Witch. by Carol Goodman

"You have only to call my name to bring me back," he whispered, his breath hot in my ear. "You have only to love me to make me human." Seduced by a powerful incubus demon, Callie has succeeded in banishing him to the Borderlands but Liam still haunts her dreams, tempting her with the knowledge of how to bring him back. But loving an incubus usually ends in death for a human. For her own sake, Callie must learn to control her desires and ensure Liam remains trapped for all eternity in his watery prison. Only there is a more dangerous creature than Liam in the Borderlands. The Water Witch is looking for a way back.