I came across a lot of debate with regards to alkaline water. I will try to give you the most important parts here though, in order that you may sift through the most vital pieces of information and not be lost in a maelstrom of sales' pitches and exaggerations.
There are 2 types of alkaline water that I was able to discern. The first is that which is artificially ionised from a metallic electrode, such as titanium. This increases the amount of electrons that are in the water and consequently gives it a negative voltage. The second form of alkaline water is that which is alkaline due to the presence of mineral ions that have an alkalising effect in the body, for example, magnesium and calcium.
Artificially ionised water can cost up to several thousand dollars to make as the device used to electrically charge the water is very expensive, and there is a filter too. However, this form of water has been linked to pathological changes in heart muscle tissue in rats.
While most humans would have to lose more than a few pounds and grow a lot more hair for this study to be completely relevant to them, it is still worthy of note as rats and humans have similar metabolic pathways within their bodies.
The above study was the reason why I didn't buy artificially ionised water.
I did, however, buy an alkaline water filter (a 2 litre jug) which filters out some chemicals normally present in tap water and then allows the water to collect in the jug where alkaline minerals such as calcium and magnesium as mentioned above, are free to diffuse into the water. Both types of alkalinisation will usually cause the tap water to reach a high pH of 8 to 11. However, I felt that the presence of minerals instead of artificial ionisation was something that the human body would be more used to in an evolutionary sense, and thus it would be better to do things the more natural way.
Again, there is a lot of debate as to the effectiveness of alkaline water. Some claim that it helps give all the benefits of an alkaline diet, all the theories of which would be too much to go into in this post but some say it is a significant aid in improving cellular oxygenation and therefore can prevent or treat pretty much every disease of civilisation and cancer too. However, this has not been proven scientifically.