Ready, Steady, Swim — Lucy Courtenay

The third book in a charming commercial chapter book series about a mermaid's adventures at her new school
 
Marnie Blue returns for another exciting year at Lady Sealia Foam's Mermaid School! This time, Marnie and her friends are gearing up to compete in the Golden Glory Day competition -- a series of events that includes a fishball tournament, an obstacle course, and a seahorse race. In the past, the winner would receive the Golden Days Crown -- but the crown went missing years ago, back when Marnie's aunt Christabel was still in school. But Marnie is determined to find the crown and swim to the rescue!