Andre Norton 
Key Out of Time 

Sent from Terra to probe the past of the deserted planet Hawaika, Time Agents Ross Murdock and Gordon Ashe are startled when murky underwater pictures of ancient races locked in a fierce battle for supremacy suddenly appear on their time-probe. Despite prophetic warnings of Karara, a Polynesian girl, the Agents decide to invade the past and uncover the secrets of this barren, lotus world.