Very. Sudden immersion causes cold shock: an involuntary gasp that can make you breathe in water, and rapid breathing for the first minute or so. Within minutes your arms and legs weaken so you can't swim. A lifejacket keeps you afloat through this.
Cold water is the main reason people drown after falling overboard, and New Zealand water is cold enough to be dangerous even in summer. The danger comes in stages.
- Cold shock (the first minute or so). Sudden immersion makes you gasp involuntarily, and if your head is under water you may breathe it in. Breathing becomes rapid and hard to control, and heart rate and blood pressure jump. Many cold-water drownings happen in this first minute.
- Swim failure (the first 10 to 30 minutes). The muscles and nerves in your arms and legs cool quickly, and you lose the strength and coordination to swim or hold on. Even strong swimmers can be unable to stay afloat or reach a boat a short distance away.
- Hypothermia (after about 30 minutes or more). The body's core temperature drops. The person becomes confused, drowsy and eventually unconscious.
What this means:
- Wear a lifejacket. It keeps you afloat through the cold shock and swim failure stages, when you can't help yourself.
- If you fall in, try not to panic or swim at once. Float on your back, let the gasping settle, and get your breathing under control. Then keep still with your legs together and arms close in to save heat, and keep hold of anything that floats.
- Get people out of the water quickly. This is why the man overboard drill is practised.
Once someone is back aboard, get them into shelter, remove wet clothes, dry them and wrap them in warm layers or a sleeping bag. Handle them gently and keep them lying down if they are very cold. Warm sweet drinks help if they are fully conscious, but not alcohol. Don't rub their arms and legs. Call for medical help if they are very cold, confused, injured or have breathed in water, since problems can develop hours later.
More in the book: Chapter 11, Safety on board