This survey addresses the immediate physiological reactions to immersion in cold water: cold shock response, diving reflex, cardiac arrhythmias and hypothermia. Cold shock response is the initial sympathetic reaction to immersion in cold water. The diving reflex is elicited by submersion of the face. Afferent and efferent nerves are the trigeminal and vagal nerves. Cardiac arrhythmias occur immediately after immersion. If the immersion persists, hypothermia becomes an issue. Hypothermia is delayed by habituation to immersion in cold water as well as insulating garments, subcutaneous fat and a large lean body mass.