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.

Preserve Body Heat when Swimming

For a more comfortable and ultimately safer swim follow these tips for conserving body heat:

Stay warm before the swim.
Heat loss is gradual and can start long before you get to the swim site.
Think of Thermal Protection.
When choosing your swimming clothes consider how many layers you would want depending on prevailing climate. You can develop hypothermia without immediately recognising it.
Keep your head warm.
You can lose up to 35% of your body heat from your head. Get used to wearing a hood before you get cold.
Reduce Windchill.
Wear windproof outer layers to keep you warm. Use unlined clothes as they won't get waterlogged if you enter the water.
Stay warm between repetitive swims.
Standing around in wet gear between swims can add to your body's deprivation of warmth through evaporative heat loss. Windproof outer layers reduce this heat loss.
Get warmed up as soon as possible after a swim.
You can start on the beach by towelling off and getting into dry clothes. If you want to jump in again later, have extra sets of dry clothes handy. It is quite uncomfortable to put the wet and cold clothes back on.
Become an educated consumer.
Visit your local sports shop and have them show you the different styles of protective garments and accessories. Don't be afraid to ask questions.

Water doesn't have to be extremely cold to cause hypothermia. Any water that is colder than body temperature causes heat loss. Water that is colder than 20°C can quickly begin to cause hypothermia. The following tips may increase your survival time in cold water, if you for a cold water swim or accidentally fall in:

Wear a life jacket.
If you plan to ride in a boat, wear a life jacket. A life jacket can help you stay alive longer in cold water by allowing you to float without using energy and providing some insulation.
Don't panic.
If you're unable to swim to safety, stay calm. Unnecessary movements require you to exert extra energy and lose body heat.
Don't remove your clothing.
Buckle, button and zip up your clothes. Cover your head if you have a hood. The layer of water between your clothing and your body will be warmed and helps insulate you.
Position your body to minimize heat loss.
Use a body position known as the heat escape lessening position (HELP) to reduce heat loss while you wait for assistance. Hold your knees to your chest to protect the trunk of your body. If you're wearing a life jacket that turns your face down in this position, bring your legs tightly together, your arms to your sides and your head back.
Huddle with others.
If you've fallen into cold water with other people, keep warm by facing each other in a tight circle.
Don't attempt to swim unless you're close to safety.
Unless a boat, another person or a life jacket is close by, stay put. Swimming expends extra energy, lowers body temperature and can shorten survival time.
Avoid excessive alcohol and drug consumption.
They may increase your risk of hypothermia. Also, don't drink alcohol and operate a boat or other watercraft. Alcohol can impair your ability to navigate the waters, increasing your risk of an accident and of falling into cold water.

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.

Cold Water Swimming Experience

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.

Reduce initial water flow

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.

Enter the water carefully

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.

Let water flow out

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.

Keep warm clothes dry

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.