The Teeth in the Tide by Rebecca F. Kenney

Pitched as Attack on Titan with mermaids, this savage gender-swapped retelling of “The Little Mermaid” is perfect for fans of Jennifer L. Armentrout, Alexandra Christo, and Holly Black.

Trapped on a walled island with her people, Kestra aches for vengeance against the swarms of ravenous mermaids that ate her father fifteen years ago. The swarms threaten her town’s last supply ship–and the life of the brave young captain who smiles so charmingly at her whenever he makes port. With the help of her scientist cousin and the handsome captain, Kestra takes her future and her people’s fate into her hands, determined to find a way to destroy all the mermaids for good.
Abused and miserable, Rake serves in the harem of the three mermaid Queens. When he is given one of his spawn to raise, Rake resolves that his little son will have a better life. With the help of a monstrous creature that deals in memories, he discovers an ancient secret—one that could lead to freedom.