Emily Windsnap And The Siren's Secret by David Kessler

A young girl who is half-mer, half-human must unravel the mystery of a dangerous siren whose song is causing trouble for both sea and shore; teaming up with friends and family, she dives into underwater politics and ancient secrets, faces dark magic and risky seas, and learns about courage, loyalty, and her own place between two worlds as she fights to protect those she loves.