The Sydney Morning Herald has printed a great explanation of the myths surrounding alkaline water and the supposed health benefits.
As the author Kent Sepkowitz says:
A more important pH to remember is this: 7.4. That’s the pH of the human bloodstream. We work very hard to stay exactly there. Every chemical reaction, all trillion or two our cells crank through daily, is optimised for 7.4. The body does not work well outside a tight range between 7.35 and 7.45; indeed much more variation and you’re liable to drop dead.
Our view has always been that the purity of the water one drinks is more important than its pH, and this article is a great explanation of how the body naturally regulates its own pH levels. Drinking water that is slightly alkaline, or acidic, has no effect on overall pH levels.