A Daring Rescue (Dolphin Island #1) by Catherine Hapka

Welcome to Dolphin Island!

Abby's family has just moved to the Florida Keys to run an island resort. She's excited to feel the surf, sand, and ocean breeze every day! And when Bella, a visitor to the island, discovers a hidden cove that's home to a pool of dolphins, Abby knows she's right where she should be.
But Bella makes Abby swear to keep the dolphins a secret. Can Abby keep her promise or will their friendship be over for good?