Andre Norton 
Key Out of Time 

Key Out of Time — the fourth book in the Time Traders series, following The Time Traders, Galactic Derelict, and The Defiant Agents — again features Ross Murdock as the hero. Accompanying a group of settlers to the now-empty water-planet Hawaika, he is sent back in time through a Time Gate . . . and must learn to survive ten thousand years in the past!