The Short Version
This claim is not supported by the scientific evidence. The highest-quality peer-reviewed studies show cold-water immersion either blunts or decreases testosterone levels in men. The only sources supporting the claim are commercial cold plunge and cryotherapy vendors with clear financial conflicts of interest, and even one of those admits no definitive clinical trial exists. Any reported increases are trivially small (~5%), transient, and within normal hormonal fluctuation — not meaningful testosterone boosts.