The Mystery of the Hidden Beach — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. But now they have a home with their grandfather, and they're spending a week in sunny Florida.

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!