Endurance, Tides of War, Book 4 by C. Alexander London

Welcome to the new Cold War. Ryan Keene and his sea lion, Hansel, are members of the US Navy's Marine Mammal Program, which sounds exciting but rarely involves any real danger. Mostly they are responsible for recovering lost objects from the ocean floor. That all changes when they travel to the perilous Arctic, where polar bears stalk the ice and killer whales prowl the water. But even at the ends of the Earth, the most dangerous preadator is man. With a team of vicious mercenaries out to get them, Ryan and Hansel are forced to flee into the frozen wasteland. They're not trained for combat, and they're not prepared to survive in one of the harshest climates on the planet. But Ryan and Hansel are a team. And they're not giving up without a fight.