Many drownings and boating related fatalities are a result of cold water immersion. When a person falls into cold water their body experiences a number of physiological responses which are affected by both the temperature of the water and the amount of time they are in the water.
Vessel capsizing and falls overboard are the leading causes of cold water immersion. Capsizing is most often caused by overloading, poorly secured loads, improper boat handling and anchoring or loss of power/steerage.
In order to prevent cold water immersion, be sure not to overload vessels and always take weather and water conditions into consideration when operating. Falls overboard can be avoided by remaining seated and limiting how much you move around a boat when it is underway.
Stages and Effects
Step 1 - Initial Immersion Cold Water "Shock"
If a person falls into cold water - their body's initial reaction is a 'gasp reflex' which can include hyperventilation and muscle spasms. This initial reaction can result in water inhalation as well as significant changes in heart rate and blood pressure. These initial effects are present for the first two or three minutes of immersion.
Step 2 - Short-Term Immersion - Impaired Function
In cold water, you may begin to experience the loss of basic motor skills after only a few minutes. Between 3 and 30 minutes after immersion a person's hands quickly lose strength and sensation and subsequently their ability to swim (even strong swimmers). In cold water immersion cases, boaters often drown as a result of swimming failure before hypothermia ever has the chance to set in.
Step 3 - Longer Term Immersion - Immersion Hypothermia
Following 30 or more minutes of immersion, hypothermia (a drop in body temperature below the normal level) will begin to set in. The persons overall body temperature will continue to drop until it reaches the same temperature of the water. As the body's core temperature falls, a person will eventually lapse into unconsciousness.
Step 4 - Post Rescue Collapse
A drop in blood pressure which may lead a person to become unconscious or to stop breathing at the point of rescue or up to several hours afterward.