In a healthy individual, the lungs and kidneys work to maintain a normal body pH. Problems with these organs can lead to either metabolic or respiratory acidosis (or alkalosis). Human blood has a pH of approximately 7.4—slightly alkaline. A fluctuation of as little as .05 in blood pH can have serious health consequences (including death) so your body has numerous checks and balances to keep that from happening. Body tissues have different pH levels. In the stomach, for example, where acids help us to digest food, the pH can go as low as 1.5, your vagina should be acidic, since yeast infections can fester if vaginal tissue becomes too alkaline. (Sorry, guys, if that’s too much information.)
Drinking water is not going to flush all of the acid out of your stomach—nor would you want it to (it’s important for things like digestion). Most of the water reabsorption happens in the intestine where your digestive tract secretes enzymes to neutralize stomach contents. So the pH and alkalinity of the water you consume is likely more or less irrelevant. By the time water gets around to affecting your blood, it has already had its pH altered several times by your digestive tract.
In practice I have noticed an increase in gall bladder issues with people consuming pH water. I’m pretty sure I know why this might be happening. Ask me!