Getting cold probably saved his life by reducing the metabolic burden. This is done intentionally during certain surgeries, such as aortic aneurysm repairs. It also informs the ER standard: you're not dead until you're warm and dead.
Also, it's staggering how much oxygen can be dissolved into your system at depth. The Navy did some experiments where they put pigs in a chamber, pressurized them to something obscene, like 300 feet, and replaced their blood with water. The oxygen diffused so well the pigs were still conscious and walking around. Source: I work with a lot of Navy dive medicine officers.
That's also why hyperbaric oxygen therapy is an effective treatment for carbon monoxide poisoning. Even when all the hemoglobin receptors are bound to carbon monoxide, enough oxygen will diffuse into the blood to keep the patient alive.
Also, it's staggering how much oxygen can be dissolved into your system at depth. The Navy did some experiments where they put pigs in a chamber, pressurized them to something obscene, like 300 feet, and replaced their blood with water. The oxygen diffused so well the pigs were still conscious and walking around. Source: I work with a lot of Navy dive medicine officers.