Lost voices trilogy

Assaulted and left on the cliffs outside of her grim Alaskan fishing village by her abusive, alcoholic uncle, fourteen-year-old Luce expects to die when she tumbles into the icy waters below, but when she instead transforms into a mermaid she is faced with struggles and choices she could never have imagined.

When humans declare war on mermaids, Luce swims to San Francisco Bay where she finds a group of "twice lost" girls, lost once when trauma turned them into mermaids and again when they defied mermaid law, and unites them into an army under her leadership.