Key Out of Time by Andre Norton

The fourth novel in Andre Norton's Time Traders series, Key Out of Time follows Ross Murdock, Gordon Ashe, Karara Trehern, and the rest of a Terran settlement team as they search the planet Hawaika for traces of a galactic civilization.All of the knowledge they had about Hawaika before embarking on their exploratory mission had been obtained from an ancient space-navigation guide. Yet, the world it described was nowhere in sight. Surrounded on all sides by shallow seas and archipelagos instead of the major landmasses they had expected to find, the Terrans must now try to reconcile the planet's past with its present. What could have transformed the planet so dramatically and what happened to the intelligent beings who had once colonized the planet? The key to this mystery lies somewhere in time.