Wim Hof is the cold water immersion world record holder. He offers a course on his techniques. Anyone have any insight? I'm skeptical of his practices as they involve hyperventilating.
breathing in and breathing out continuously (30-40 times) followed by an exhale breath hold. After every cycle take a deep breath and hold it for another 10-15 seconds (if you want longer, it’s not a problem at all).
I'm considering trying this with a standard free dive relaxed breathe-up instead.