Hypothermia
Most victims die from cold shock and incapacitation long before hypothermia becomes an issue.
Before a person becomes hypothermic, they will be cold, miserable, and shivering violently.
Low Body Temperature
Hypothermia means low body temperature. It's medically defined as a core temperature below 95F (35C). (Your body core is basically your entire body minus your arms and legs.) You have to survive both cold shock and incapacitation before hypothermia becomes an issue.
Because of their smaller body size, children cool much faster than adults. It takes about thirty minutes of immersion for an adult of average size to develop hypothermia – even in near-freezing water - but a very large person with a lot of body fat can delay both physical incapacitation and hypothermia, sometimes for hours.
A Strong Defense
When your body is having trouble keeping warm, it mounts a strong and automatic defense against the cold. This defense has one purpose: to preserve the core temperature and protect vital organs like the brain, heart, lungs, kidneys and liver.
First, circulation is reduced to the skin surface and to the hands and feet. If that's not enough, blood flow to the arms and legs is reduced. Shivering also begins. Shivering automatically increases heat production through involuntary exercise, and it's an important early warning sign that you're losing too much heat. At this point, you aren't hypothermic, but you're heading in that direction. Shivering grows more intense as body temperature begins to fall.
Drowning and Hypothermia
As body temperature falls below 95F (35C), both mental ability and consciousness suffer, and like cold shock and incapacitation, the primary danger is drowning.
Picture yourself floating helplessly in the water. Your mouth is roughly three inches above the surface - and that's when the water is calm. Your arms and legs don't work, and the only thing keeping you afloat is your PFD (Lifejacket). The kind that most people wear isn't designed to keep you floating face up if you're incapacitated or unconscious.
As your core temperature starts to fall, you become progressively more confused and disoriented because your brain is becoming colder. It takes more and more effort to keep your head upright and hold your breath to avoid inhaling water if waves are splashing your face - and you're losing that battle.
Badly Chilled or Hypothermic
How Can You Tell?
It can be very difficult to tell the difference between someone who is badly chilled and someone who has hypothermia. Because of the physical and mental effects of cold shock and incapacitation, many hypothermia symptoms can be present in people who have a normal body temperature. From a practical standpoint, all you’ll have to guide you are the environmental circumstances and the appearance and behavior of the victim.
In other words, it’s quite possible for a badly chilled person to have difficulty thinking and speaking clearly, and to also be physically incapacitated, shivering violently, and unable to use their hands. For this reason, emphasizing concepts like the "umbles" is unlikely to be of much practical value. The same goes for having students in wilderness first-aid classes memorize the "stages" of hypothermia.
Diagnosis
The only reliable way to determine if someone has hypothermia is to measure their core temperature, something that can’t be done orally. If you have an electronic thermometer that reads below 95F (35C) and is designed to measure temperature via the ear, that's an effective method.
A Difficult Situation
Regardless of whether the person is badly chilled or hypothermic, you’re going to have a very big problem on your hands because neither situation is easy to deal with in the outdoors.
Exercise
However, if the chilled person is still out on the water – for example, after being rescued and getting back in their paddlecraft – it's important for them to continue to paddle if they can manage it. That's because exercise generates body heat and will help them from getting any colder. This is a particularly important consideration if they're being towed.
Towing
Furthermore, if you have to tow another kayaker and they're unstable, a very efficient tow rig can be quickly constructed by lashing a spare paddle to their deck and putting paddle floats on the blades so that the system serves as outriggers to prevent a capsize.
Not Just an Immersion Issue
You can become badly chilled or hypothermic even if you never enter the water. Whenever body heat is lost to the environment faster than you can replace it, incapacitation will be followed by hypothermia.
In cold, windy, rainy weather, a person exposed to the elements can quickly become incapacitated while paddling or making camp. Again, the first things to go are your hands. Feeling cold, having cold or numb hands, and shivering are all early warning signs that should be taken seriously. If you're in a group, ask people if they feel warm enough. If they don't, take steps to correct the problem before it gets any worse.