Just for Fins by Tera Lynn Childs

Tera Lynn Childs brings a delightful mix of wit, romance, and fantasy to her books. Fins, her YA mermaid series, is a great pick for fans of humorous love stories. The perfect beach read!
 
In Just for Fins, third in the series, half-human, half-mermaid Lily Sanderson has decided to remain Crown Princess Waterlily of the mermaid kingdom Thalassinia. She’s made a promise to help to help Prince Tellin.

Climate change and human pollution are causing suffering in his kingdom, and Lily and Tellin have to convince all the mermaid royals to work together to save the oceans.
 
Lily is distracted by the situation with Quince, the boy she loves. He must pass three tests to prove his love for her or they will be parted forever...