When we swim, the water removes the heat out of our body. We can also say that our body tend to equilibrate with the temperature of the water. When the swimmer get out of the water, the remaining water in the body tend equilibrate its temperature with the air. The consequence is that, the water in our body will absorb some heat from the swimmer's body thus making himself very cold.
We can simply state it as: the evaporation of the water in the skin of the swimmer take some of the heat from the body thus cooling it (the body) at the same time.