Key Out of Time

In the present day, Time Agents Ross Murdock and Gordon Ashe come with settlers to the water-dominated planet, Hawaika, to search remains from the distant past of the alien Baldies. They are assisted by intelligent dolphins, with whom the humans can communicate. While setting up their time gate, a storm destroys it and strands them in widely separated locations in Hawaika's unknown past.
Murdock learns from a native named Loketh that Gordon is being held in a castle. Murdock and Loketh are captured by seafaring Rovers, then join them. They liberate a Rover island that had been captured by the Baldies. Murdock convinces a coalition of natives that the Baldies are playing them against one another.
Murdock finds Ashe at last, in the company of the mystic and advanced Foanna, of whom there turns out to be only three – the last of their race. The Foanna set a trap for the Baldies, using their castle as bait, but with only three of them, they cannot win against the whole attacking force. Murdock and Ashe agree to a process that mentally joins them with the Foanna, and in a second encounter with the Baldies, they win.
In a final encounter, Murdock is teleported into a Baldy ship like the one familiar to him from Galactic Derelict, and sets its course to a random destination. The main Baldy installation is simultaneously attacked and the Baldies driven off the planet.