Immersion in cold water represents one of the greatest environmental stresses to which the body can be exposed. Drowning is the leading cause of death in those undertaking sport, including recreational diving, and immersion is the second most common cause of accidental death in many countries of the world. However, it is a relatively “hidden” killer with many of the 1,000+ immersion deaths that occur each day worldwide going unnoticed. Drowning is also a “disease of youth”, 64% of deaths are < 30 years old; 25% are < 5years old. Drowning death, in terms of time to cardio-respiratory arrest after submersion, takes about 130 seconds; with the chance of successful resuscitation falling to nearly zero percent by 27 minutes. This time increases to around 66 minutes if water temperature is below 6 °C (Tipton & Golden, 2011). The likelihood of both drowning and surviving is intimately related to the change in the thermal state of the body. “Change” is a powerful stimulus to homeothermic animals. The hazardous physiological responses to cold water immersion that can be precursors to pathophysiological consequences such as drowning and sudden cardiac arrest are driven by the rate of change (fall) of skin temperature and, in terms of the cardiac response, the resulting change in sympathetic and parasympathetic inputs to the heart (Tipton et al. 1991; Winter et al. 2019). The physiological responses resulting in prolonged survival underwater are also dependent on rate of change, but this time it is the rate of change (fall) of brain temperature (Tipton & Golden, 2011). Which of these outcomes prevails on submersion: death or survival, is also influenced by a wide variety of factors that we are only just beginning to understand and, thereby, tentatively predict those at most risk.