The science behind blood pH

You can't change the pH of your body by drinking alkaline water. Your body regulates its blood pH in a very narrow margin. If your pH varied too much, you wouldn't survive. 

However, your diet does affect the pH of your urine. Most people's urine is about 6, which is acidic and not a problem. Alkaline may make your urine less acidic, but it doesn't make a difference to your health.