Alex Hutchinson, Outside:

In February 2006, Robert Hewitt was scuba diving near Mana Island, off the coast of New Zealand’s North Island. Hewitt was an experienced navy diving instructor with 20 years in the service, and he told his dive buddy that he would swim back to shore himself. Instead, when he next surfaced, he had been pulled several hundred meters away by a strong current. The dive boat had moved on, and Hewitt was left alone, the tide pushing him farther and farther from shore.

And:

The most pressing challenge facing Hewitt was the water temperature of 61 to 63 degrees Fahrenheit (16 to 17 degrees Celsius), well below body temperature. According to physiological models, when water is 59 degrees Fahrenheit (15 degrees Celsius), the median survival time is between 4.8 and 7.7 hours. Amazingly, Hewitt spent the next 75 hours in the water, drifting back and forth over a distance of nearly 40 miles.

And:

In general, immersion in cold water produces a four-stage response. First is the “cold shock response” that triggers “an inspiratory gasp, uncontrollable hyperventilation, hypertension, and increased cardiac workload.” If you’re not ready for it, this shock response can cause you to inhale water and drown and can set off heart arrhythmias.

Riveting, riveting read. And if this interests you, watch this video.