Merman's Bond — Dee J. Stone

After fighting off the rebel merpeople holding them hostage in the ocean, Cassie and Damarian finally make it back to land, to safety. Now they need to learn how to balance both their human and merpeople lives. Staying in the sea for too long causes Cassie's friends and family to worry. But how can they remain on land when the merpeople colony is threatened by the rebels? As if that's not enough, there are changes going on in Cassie's body, changes that seem to have nothing to do with her mermaid side.

It's not long before they learn the truth: Cassie's very existence threatens to bring an end to the entire merpeople race. Can she and Damarian figure out how to stop it before they're torn apart forever?