This is book number 41 in the The Boxcar Children Mysteries series.
The children are attending Camp Coral, where they learn about the ocean and all the things in it — coral, fish, dolphins, and other sea creatures. They find a beautiful hidden beach, but then learn that it holds a terrible secret. Once again, the Boxcar Children must come to the rescue!
The Boxcar Children Series was created by Gertrude Chandler Warner, a teacher, when she realized that there were few, if any, books for children that were both easy and fun to read. She drew on her own experiences in writing the mysteries. As a child, she had spent hours watching trains near her home, and often dreamed about what it would be like to live in a caboose or freight car. In each story, she chose a special setting and introduced unpredictable, unusual or eccentric characters, to help highlight the Aldens’ independence and resourcefulness. Miss Warner lived in Putnam, Massachusetts until her death in 1979.