Sudden cold water immersion drastically reduces your ability to hold your breath typically from a minute or so to less than 10 seconds, whilst cold water in your ears can cause vertigo and disorientation.
At a water temperature below 15°C, and if you are not wearing a life jacket, especially an automatic one, cold water shock will:
cause you to inhale as you go under the water, due to an involuntary gasping reflex, and drown without coming back to the surface
drastically reduce your ability to hold your breath underwater, typically from a minute or so to less than 10 seconds
induce vertigo as your ears are exposed to cold water, resulting in failure to differentiate between up and down
Cold Water Shock is a danger in water below 15°C; that’s more or less the summertime average around the coast of the UK. It is therefore important to think carefully about the clothing you wear and protection from the cold – a dry suit will provide additional protection, particularly in very cold water.
Coupled with the shock of going over the side, the condition may well contribute to a feeling of panic as you struggle to stay afloat; this will be far easier to overcome if you are wearing a correctly fitting lifejacket/ buoyancy.