A number of reasons are cited such as water diluting our stomach acid or water pushing undigested food out of the stomach.
Here is the truth to the age-old myth
When we chew food it mixes with the saliva which contains the necessary enzymes for digestion. Then the softened food gets into stomach which gets mixed with the stomach acid. Average time needed for digestion is four hours.
It takes nearly 10 minutes for 300 ml water to move from the stomach. According to reports, drinking water while eating goes through chewed food quickly, moisturises it and leaves the stomach quickly.