If the body is unexpectedly immersed in water colder than 15°C, a number of involuntary reactions happen. The diving reflex, that usually protects you from inhaling when the skin around your nose comes into contact with water, fails. Instead, you gasp in a deep breath – and might well drown from the first water intake.
Subsequently, you will lose control over your breath and specifically the capability to hold your breath. If you usually are able to hold your breath for about a minute, cold water shock will get that down to 10 seconds or even less, leading to hyperventilation. The cold water on your skin will make your blood vessels contract, which leads to higher blood pressure and a considerable increase of your heart rate. Cold water in your ears might lead to vertigo and disorientation. You will lose feeling in your extremities and may not be able to move. All these effects easily lead to panic which further accelerates the physiological effects. With time, the body adjusts to the cold water and you will be able to breathe again and move. However, it is critical to survive these first few minutes. If you happen to fall into cold water, try to stay calm and don’t move too much until you get your breathing under control.