Mermaid’s Awakening by Dee J. Stone

All her life, sixteen-year-old Marella Sapphire felt like she was different from everyone else. But the last thing she expects is to turn into a mermaid.
It turns out her parents have been hiding a huge secret from her. They’re both merpeople and now she is one, too. And there’s a whole colony of merpeople deep in the ocean—her family—waiting to meet her. Her parents expect her to pack up her life and go down there. Live as a mermaid, like that’s totally normal. But Marella doesn’t want this. She misses her old life, her friends, and the boy she has a crush on.
After making a deal with her parents to give it a shot, she spends some time in the ocean. She meets her family and makes some friends. She realizes that life in the ocean might not be that bad after all.
Until a danger threatens the ocean. And Marella might be the key to saving it.