Deep Blue by Jennifer Donnelly

Uploader's note: This is a very early copy. It has some issues.

Deep in the ocean, in a world not so different from our own, live the merpeople. Their communities are spread throughout the oceans, seas, and freshwaters all over the globe.

When Serafina, a mermaid of the Mediterranean Sea, awakens on the morning of her betrothal, her biggest worry should be winning the love of handsome Prince Mahdi. And yet Sera finds herself haunted by strange dreams that foretell the return of an ancient evil. Her dark premonitions are confirmed when an assassin's arrow poisons Sera's mother. Now, Serafina must embark on a quest to find the assassin's master and prevent a war between the Mer nations. Led only by her shadowy dreams, Sera searches for five other mermaid heroines who are scattered across the six seas. Together, they will form an unbreakable bond of sisterhood and uncover a conspiracy that threatens their world's very existence.