My dad seems to think that it it dilutes the gastric acid that digests the food in our stomach making it less than optimal.
I want to believe him but somehow it doesn’t make a lot of sense to me. What really happens if we drink a big cup of water just before or just after we eat?
In short, yes drinking water dilutes stomach acid. However, the stomach is capable of detecting and correcting this change (as it tries to keep itself balanced at a certain pH). Even if you drink a liter of water directly before eating, the change in pH of your stomach acid will be correct quickly enough to make little difference in digestion. Alternatively, drinking water before or with a meal is known to promote other positive health effects that are more beneficial than the detrimental affects of the minor, temporary increase in the pH of your stomach acid.
So yes, water dilutes gastric acid, not significantly enough or for long enough to affect digestion. Drinking water with food (and just generally staying hydrated) is significantly better than not doing so.
Your father is not really being accurate. Water goes through the stomach incredibly quickly, and essentially doesn’t stay there for any length of time at all. And it is absorbed by the small intestines very quickly too. It only takes a few minutes for most of the water you drink to be in your bloodstream.
The only difference it would make between drinking water before and after is that the process of digestion requires a pretty good bit of water itself, so it dries you out a bit. So if you were on the verge of dehydration, then you would want to drink before you ate. But unless you are nearly dying of lack of hydration right then, it isn’t going to really make a difference.