Andre Norton - Key Out of Time (version 2)

Time Agents Ross Murdock and Gordon Ashe return for another assignment, this time to the tropical island paradise planet of Hawaika. Something apocalyptic has occurred in this planet’s history, altering its topography and cleansing it of all intelligent life. But something goes wrong when they attempt a peek at the past through a Time Gate, and with the Polynesian girl Karara and her trained dolphins Tino-rau and Taua, are plunged into a conflict between a dying super-race, barbarian natives, and invading aliens. (Summary by Mark Nelson)