The graph explains and shows how someone would swim. The speed when swimming is constant for how fast your going. It depends on what swimming technique you are using. If you are swimming a freestyle you for sure are going to be using a constant speed, but if it was something that you where told for you too such as your teacher or so on swim you most likely wouldn't swim at a constant speed.

The acceleration in the graph is the increasing speed of the swimmer. This is a example of no constant speed. The swimmer is going faster as the time is getting smaller on the graph.

Work: Swimming is a great total body workout that incorporates many different muscles and muscle groups. Each stroke uses different muscles. It is important to work these muscles both in and out of the water to become a strong and injury-free swimmer.
Power: Is equal to force times velocity. Force is measured in pounds and velocity is measured in feet per second.

When you are swimming you are using kinetic energy. Anything that is currently moving, it is using kinetic energy. You will also have more kinetic energy depending on your mass and speed.
Stored potential energy in the body is converted into kinetic energy to be burned by the muscles activated in swimming and maintaining body temperature in cold or cool water.