It is said that after dinner or after lunch , one should not take a cold water bath. Due to the cold water flowing on the body, the blood circulation increases around the body near the skin. This reduces the blood flow in the stomach. So digestion process is affected.
Also, cooling the body by taking bath is not good, when energy is being produced by the metabolism inside the body and when body is being warmed. One can experience it by oneself easily, it feels strange.