We will feel cold when jumping into a pool as the skin temperature will drops rapidly. We will lose heat through our skin to the pool. After a few minutes, we will not feel cold as our body temperature will be similar to the pool temperature.
We will also feel cold when getting out of a pool. The body produce heat to keep us warm while we were in the pool. Once we leave the pool, there will be water on the skin. The body heat is transfer to the water. The water droplet will gain energy and evaporate. Due to the evaporation, our body will lose heat and we will feel cold as a result.