Above the water our body does not use much energy to regulate its temperature. In the water we spend a considerable amount of energy to keep warm which leads to tiredness and dehydration. Add windchill and you'll see the challenge.
Your body loses heat in water about 26 times faster than in air. If the water is moving around the body the heat loss is much higher. In other words, if the water is not perfectly calm but is moving, even by small amounts of current or small waves, you'll chill out faster. Same applies when you move through the water.
Also, the top thermal layer of "warm" water, even in the hottest days of summer,
does not extend far into waters that are deeper than your own height.
Dressing to stay warm is all about slowing the transfer of heat from your body to the outside. This can be described quantitatively using the physical law of heat conduction.
H is the amount of heat energy per time unit that moves from your body to the outside.
A is the surface area of your body.
dT is the difference in temperature between your body and the outside.
dx is the distance from your skin to the outside (clothing layers).
k is a constant determined by the insulating material.
The k (thermal conductivity) of water is 0.6 and the thermal conductivity of air is 0.023. From this (0.6/0.023) you can see that the conductivity of heat through water is about 26 times greater than through air.
Dry fleece is mostly trapped air and has a thermal conductivity of about 0.08
whilst cotton saturated in water is mostly water and will have a thermal conductivity close to the 0.6 of water.
For a more comfortable and ultimately safer swim follow these tips for conserving body heat:
The dangers of cold water shock should be taught in school swimming lessons,
or during PSHE lessons if schools do not arrange swimming lessons for pupils.
Sometimes it may be necessary to swim in clothes. You may be on a sinking boat, or you are cut off by a rising flood or tide. Maybe you simply want to cross a stretch of open water that is in your way or you just fancy a swim outdoors. Whatever may happen, you should practise and prepare for it.
Before you enter the water tighten neck openings and cuffs. This traps air for buoyancy and slows the water soaking your clothes underneath. Eventually the water will find its way in, but you can delay that. How long the water takes to fully saturate your clothing depends on your movement in the water and how watertight your waterproof clothes are at neck, cuffs, waist and leg. On short crossings that can make a difference.
You can slow the water coming in if you move carefully. Keep your clothes tight as long as you have buoyancy for air pockets. You may notice that you float up higher than usual. Eventually the water will seep in anyway.
Once your clothing is fully saturated, loosen it a bit to allow the water to flow out so it doesn't collect anywhere. The action of swimming tends to cause water to collect within your outer clothing, in sealed cuffs, sealed ankles or boots. This somewhat restricts movement and slows you down.
Cold water will rapidly cause fatigue. Hypothermia will be a major factor to consider prior entering the water.
Pack away your relatively dry warm undersuits before your swim. Put them inside a waterproof bag for the duration of your swim.
If a fibre pile or fleece suit gets saturated it provides little or no thermal insulation.
Take it off and wring it out to expel as much water as possible before you put it back on.
This reduces the risk of subsequent hypothermia.