Last year's Lost Voices by Sarah Porter has a sequel in the recently published Waking Storms.
Here's what the publisher had to say about the first one:
"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. "
And what they have to say about the second in the eventual trilogy:
"After parting ways with her troubled mermaid tribe, Luce just wants to live peacefullyon her own. But her tranquility doesn’t last long: she receives news that the tribe ison the verge of collapse and desperately needs her leadership. The tribe’s cruel queen wants Luce dead. Dorian, the boy Luce broke mermaid law to save, is determined to make her pay for her part in the murder of his family. And while the mermaids cling to the idea that humans never suspect their existence, there are suddenly ominous signs to the contrary."