The Great Shark Mystery (Box Car Children) by Gertrude Chandler Warner

The Alden children are on vacation at an aquarium in Florida. The Boxcar Children will feed the penguins, swim with dolphins, and come face-to-face with a great white shark! The shark draws crowds and thrills the park's visitors. But the Aldens soon discover that the shark is in danger―someone at the park wants it gone. The Boxcar Children are determined to fish out the crook before it's too late for the shark.