This book first of all deals with suicide, self harm, depression, and anxiety. It talks about these mental health issues in an interesting way. The author puts a note in the beginning for these triggers and also states "you are seen, you are loved, you are not alone"

Coral is a mermaid who lost her older sister to the Disease. She wants to escape her world and with the help of her grandmother, does.
Brooke is a girl living in a group therapy home with her own depression and tries to end it all. She's saved by a boy.
Merrick is a rich boy who wants for nothing... Except to get him and his sister, who's already tried to end her life, away from their seemingly uncaring and uncontrolling father. They also have their mother who abanfons them-thing that was best.

The three lives are intertwined throughout the novel. I admit it took me a long while to get how they were but it became clearer and clearer as the story goes on. It all comes together with all 3 helping each other in ways they never thought possible. Very well done.