Key Out of Time by Andre Norton

Ashe Gordon and Ross Murdock, infuriated about the loss of their fellow American Time Agent, Travis Fox, on the planet Topaz, travel to a warm ocean planet, Hawaika, where they intend to set up a time gate. The world is so different from what they expected that they decide on a risky experiment: travel into the past of the planet, accompanied by two mutant dolphins and a female agent of Polynesian ancestry.

Murdock gets trapped in the ancient past of the water world of Hawaika, facing terrifying wizards in a kingdom he knows will soon be utterly annihilated by an alien empire that is bent on the conquest of the entire galaxy. The fates of two worlds, and possibly the galaxy itself, will be determined by the actions of these castaways in time - and whatever happens, the lives of Time Agents everywhere will be changed forever.

You can view more about this title at the CruGuru website. It is available from Amazon.com. ISBN: 978-1-920265-21-2