The answer, from Daniel Layon, Medical Student, Class of 2017: The amount of water you drink during a meal doesn’t really matter.
It won’t hurt (unless it’s a lethal quantity of water) and it doesn’t help (unless you are already dehydrated).
Digestion is a technical term which relates the mechanical, enzymatic, and chemical degradation of food. Almost every single physiological process in your body takes place in an aqueous, relatively neutral environment, so saying that drinking normal amounts of water during a meal will help / hurt digestion is nonsensical.
The common belief that many people have is that water will reduce the acidity of your stomach acid, which for all intents and purposes is not true. You cannot dilute your stomach acid in any physiologically meaningful way (e.g. HURT the digestive system)1 by drinking water during a meal. The pH of stomach acid is <1. That means your stomach acid is 100,000 x more acidic than water (pH of ~7). You would have to be drinking LITERS of water to dilute your stomach acid in any meaningful way.
Furthermore, there is not just a “pool” of acid in your stomach. Gastric distention (stretching of the stomach), and chewing, seeing, smelling / or THINKING of food increase production of gastric acid.”
[This question and answer was originally published on Quora. See the other responses here.]