Gentleness from author of 'Jaws'; The Girl of the Sea of Cortez, by Peter Benchley. New York: Doubleday & Co. 236 pp. $13.95.

A book about stunningly beautiful hammerhead sharks from the man who gave us ''Jaws''? A book about frolicsome moray eels from the man who gave us terrifying denizens of the rocks in ''The Deep''? Has Peter Benchley, author of underwater adventure stories extraordinaire, gone soft as a jellyfish?

Probably not. But he has certainly found a new voice in this tender new novel.

It apparently began with a photo assignment for ABC Sports in the Gulf of California, known locally as the Sea of Cortez. One of Benchley's assistants spotted a king-size manta ray lurking beneath the boat one afternoon and dropped overboard for a closer look. After pulling some fishing lines from a wound in the ray's skin, she held on for a few more minutes and was taken for a galloping ride. For the next three days everyone in the crew climbed aboard for rides.

That real-life experience is the basis of Benchley's novel, and the book is packed with plenty of fascinating sea lore. We read about the formation of seamounts, learn where pearl oysters grow, and get a glimpse of the trust that can exist between man and the unfamiliar creatures of the deep.

But when ''The Girl of the Sea of Cortez'' leaves fact behind and cruises into uncharted realms of human relationships and ancient superstitions, it sparkles like a parrotfish.

In the course of teaching his teen-age daughter how to dive, a fisherman also helps her discover her own self-worth. A lonely girl who's shunned by the other women of the island because she spends her days diving on the seamount instead of washing clothes, Paloma has a lot of the ''good thing'' her father tells her animals respect and trust. Father and daughter share a special closeness, and when he's killed at sea, a rift develops between Paloma and her jealous younger brother that threatens to end in tragedy.

In some ways, it's hard not to compare Benchley's tale of a young girl and the sea with Hemingway's classic ''The Old Man and the Sea.'' Though the writing is similarly clean and straightforward and the characters simply drawn, this will probably be a once-read story, not a book to keep pulling off the shelf.

Implausible at times, it's nevertheless endearing. Given just the right spot under just the right shade tree, many readers should find some pleasant summer reading here.