A tender story about a frisky baby sea otter; Song of the Otter, by Edith Thatcher Hurd, illustrated by Jennifer Dewey. San Francisco: Sierra Club Books (distributed by Pantheon Books, New York.) 35 pp. $ 8.95. Ages 8-10.

The sea otter, now nearly extinct, is one of nature's stranger denizens. Bobbing in the ocean currents, this curious creature with its furry body, whiskers, and front paws looks more like a land animal than a water dweller. But look beneath the surface and you'll find the reason for this oddity - his strong back flippers bind him to the sea.

In this book, children will learn about the habits of this unusual creature through the tender story of a curious baby sea otter as he explores his new environment. Voyaging out into the vast ocean and plunging to the bottom, the baby learns how to survive without its mother. He meets other sea animals. Yet one question remains unanswered for him: What is land like?

Washed ashore one day by a storm, the creature finds the answer. ''Walking on rocks was not like swimming. He could not twist and turn and do somersaults, the way he did in the water,'' writes the author. Soon the otter returns to his natural environment, satisfied to know it is where he belongs.

The clear charcoal drawings, found on nearly every page, are captivating. With this book the Sierra Club has succeeded in helping young readers care more about this endangered species.