BOATsmart! USA Knowledge Base

Cold water immersion occurs when a person experiences sudden, unexpected immersion into cold water. If this happens to one of your passengers, their body will experience the following four phases of physiological impact:

1. Initial reaction (cold shock):

This phase lasts for about one minute. A deep and sudden gasp will be followed by hyperventilation and involuntary gasping. These actions can cause the person to inhale water and drown. During this time, the person overboard must avoid panicking and concentrate on controlling their breathing. Wearing a lifejacket at this initial phase will be critical to their survival.

2. Short-term immersion (swimming failure):

This phase lasts for about 10 minutes. During this time, the person overboard will lose the use of their fingers, arms and legs. Swim failure will occur within these critical minutes and if the person overboard is not wearing a lifejacket, they will likely drown.

3. Long-term immersion (hypothermia):

The next phase is when hypothermia sets in. It can take up to an hour before the person will become unconscious. During this time, the body organs will be cooling and the internal temperature of the body will be dropping. Following the phase of unconsciousness, if not rescued and treated immediately, the person overboard will likely die.

4. Post-rescue collapse:

Once rescued, the person will experience a phase during which their body is under extreme stress and at risk of cardiac arrest. The arterial blood pressure will change and the cold blood from their limbs will re-enter the internal body core, causing dangerous stress on their heart and the brain.

Note: Always wear your lifejacket and properly secure it—especially if you’re boating in cold weather or in cold water areas. If a lifejacket is thrown to you once you’re already in the water, your muscles may already be paralyzed from the cold and you may be unable to secure the lifejacket you’re wearing or to reach for a flotation device that is thrown to you.