From an energy consumption point of view, resistance in the water for a person is 12 times greater than that on the ground.  Therefore, the completion of exercise exhausts much more energy in the water than on the ground for the same distance.  The energy consumption of swimming 200 meters is equivalent to running 1500 meters on the ground.  Moreover, heat conduction velocity in the water is 12 times greater than that in the air.  Coupled with the effect of water temperature, the energy consumption of the human body is relatively great, which is one reason why many people's appetites are greatly increased after swimming.