Call of the Fathoms by Emma Hamm

The City of Tau has long been a secret. Until now. As a genetic experiment and personal guard to one of the Originals, Alexia has always known her purpose. Keep her Original safe. Ensure the city of Tau remains hidden amongst the depths of the ocean. And destroy any and all undine who try to make those jobs difficult.
Fortis has always known he was going to die early. His wife had seen a prophecy of his death in great detail, so he knew the exact moment he would expire. Which meant he wasn’t afraid. Fate could not be changed, not even by the warrior woman who captures his attention. After all, he knows the hour and circumstances of his death.
He lures her into hunting him, but he did not expect to enjoy their battles as much as he did. This woman has much to learn. If he had more time, perhaps he would make a good teacher for her. But he needs to convince this warrior to help them, even though she hates his kind.
The longer they are together, the more they both realize that there is not so much different between their species after all. And when Alexia discovers there is a great deception in her own city, she realizes perhaps it would not be so difficult to help the undine after all. Or… to even fall in love.