The Book of Water - The Dark Library Series, #1

The merfolk have always been at war-with humans, with the sea, and with each other. Celine's war lies within, the despair from her parents' murder sending her to the open arms of the Sea Witch, Kasindra, whose dark magic soothes the burning need for revenge against the merking who usurped the throne. But the merking forces Celine to change, to marry the human prince of Estilon to unify their kingdoms and end the war between Estilon and their merfolk kingdom. Owen, the human prince of Estilon, also wants revenge.
He blames the merking for the mysterious death of his father, who died in agony under the thrall of a merfolk toxin. Together, Celine and Owen form an unlikely alliance, but as they get closer, they must navigate the treacherous waters of love and betrayal. In this dark fantasy retelling of The Little Mermaid, Celine and Owen will risk everything to seek their revenge-but their hearts may be the price they pay.