For swimmers, about 91 percent of their energy is lost through drag.  Swimmers need to reduce their surface area and maximize streamline when they travel in water.  When their surface area is reduced, as is their resistance and their drag.

Swimmers also use a combination of kicking and pulling in order to propel their bodies through the water.
Another important factor in swimming is buoyancy.

In order for a swimmer to reduce drag, they must keep their body straight and symmetrical. This is commonly known as streamlining. The swimmer keeps their legs tightly together, their arms stretched in front of them touching, and their biceps next to their ears. This limits drag by limiting the frontal area of the swimmer in the direction they are moving in.

http://swimming.about.com/b/2010/11/03/streamline-to-faster-swimming-swim-faster-the-easy-way-streamlines.htm

Buoyancy is needed to keep swimmers afloat. In order to establish greater buoyancy, swimmers try to press their chests down which causes the hips to rise. This is called “pressing the T”. By “pressing the T”, swimmers become parallel to the surface, which causes them to float.