The whole alkaline food/water thing is complete bullshit. Look, the pH of your stomach is naturally about 1.5-3.0. Remember that pH is a logarithmic scale. So, the acidity of your stomach (say, pH 2) is naturally about 10,000 times higher than your "acidic" (pH 6, let's say) tap water. If you drank 10 litres of "alkaline water" (say, pH 8) in one quick sitting, you would only raise your stomach pH by 1.0 unit. You would die from drinking so much water at once, however. What about acidic tap water? Let's assume your tapwater is at pH 6 (BTW, pure rain water from an unpolluted atmosphere is even more acidic with a pH of 5.6). If you drank 10 litres of that you'd still increase your stomach pH 1.0 unit (OK, 0.9999 units). The acidity of your tap water (even the most acidic) is nothing compared to the acidity of your stomach.
There are many scams about water treatment and filtration that take advantage of the scientific ignorance that most people have. Magnetic treatments of water hardness is a fun example. Sometimes things get criminal: plumbers are selling household filters to remove lead from tapwater in Flint MI (you can't filter out dissolved lead...).