Viruses and disease microbes flourish under acidic conditions, so drinking alkaline water helps prevent disease. Acids also eat away at mucous linings. When I drink too much coffee/green tea and eat too many grains, I start getting cold sores in my mouth. If I don't stop, they multiply. When I do stop, they go away within a day or two. I've learned to tell when I'm just starting to get one, so I can stop right away and the cold sore doesn't mature.
I can only imagine how stomach and intestinal linings are affected. They're built to resist the acids we produce to digest food, but what happens when we add to that acidic content by drinking much-needed water that is also acid? Could that be one of the causes of ulcers?