What energy is used in swimming?

In simplest terms, the metabolic energy, or the energy produced through the process of metabolism, is used in swimming. This energy is converted into the mechanical energy that the swimmer needs to overcome the water resistance. The energy that is utilized is in the form of ATP molecules.
There are three different energy systems that come into play during the swimming:
ATP-PC system
Lactic Acid system
Aerobic system
Initially, when the swimmer pushes off the wall or jumps into the swimming pool, the ATP-PC system provides the energy for about 10 seconds. Afterwards, the lactic acid system takes over and this anaerobic energy system provides the energy for the next 30 seconds to a minute. Finally, if you continue to swim beyond this time duration, the aerobic system will take over and provide the necessary energy. 
The contribution of each of these energy systems is dependent upon the duration of swimming and has been studied by researchers. For short duration events (such as 50 m sprint), the ATP-PC system will provide most of the energy, as compared to a relatively longer event (such as 200 m freestyle), where the aerobic system will be the chief contributor to the total energy requirements.
 
Hope this helps.