Swimming relies on the natural buoyancy of the human body, with the relative density of the average body, compared to water, of 0.98, creating a floating effect. This varies on the basis of body composition, with body fat lowering the density, and increasing floatation.
The relative density difference means that water supports the body during swimming, and therefore makes swimming low impact compared to surface activities such as running where weight is put on to the joints, and also creates resistance when moving through the water. The resistance is used by swimming strokes to create propulsion, but creates drag on the body.
This means that hydrodynamics are an important factor in stroke technique in terms of swimming faster, and swimmers wishing to swim faster, or wishing to tire less will try and reduce the drag caused by the body through the water. In order to be more hydrodynamic, people can increase the power of the strokes, or reduce water resistance, although increasing power to overcome resistance needs to increase by a factor of three to achieve the same effect as reducing resistance.
Efficient swimming by reducing water resistance involves having a horizontal water position, rolling the body in order to reduce the breadth of the body in the water and extending the arms as far as possible in order to reduce wave resistance.
Read more about this topic: Human Swimming
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