The Shark Rider — Ellen Prager

After thwarting the dastardly plans of J.P. Rickerton, Tristan Hunt is having trouble keeping his newfound talents a secret. And if undercover spies and a mysterious illness threatening to expose the secrets of camp weren't enough, reports of dying fish and disappearing sponge in the Caribbean call Tristan and his friends back into action. Will the Sea Guardians discover the source of the problem before time runs out? Can the escape the threat of an oncoming storm? Or will a betrayal from one of their own ensure it's already too late?