The Blue Lagoon by H. De Vere Stacpoole

At noon, in the shallows of the reef, under the burning sun, the water would be quite warm. They would carry the baby down here, and Emmeline would wash it with a bit of flannel. After a few days it scarcely ever screamed, even when she washed it. It would lie on her knees during the process, striking valiantly out with its arms and legs, staring straight up at the sky. Then, when she turned it on its face, it would lay its head down and chuckle and blow bubbles at the coral of the reef, examining, apparently, the pattern of the coral with deep and philosophic attention.

Dick would sit by with his knees up to his chin, watching it all. He felt himself to be part proprietor in the thing -- as indeed he was. The mystery of the affair still hung over them both. A week ago they two had been alone, and suddenly from nowhere this new individual had appeared.