I think the better question should be: Is Alkaline Water Safe? This is going to be the start of an ongoing discussion and several sources will be listed:

Dr. Mercola has this to say about alkaline water:

Many alkaline water enthusiasts are convinced its powers are unparalleled and will vehemently defend it. I am also certain that many will post vigorous objections to my position, and that is their choice. It is also my choice and responsibility to provide information on a system that many people are relying on to provide health benefits that I feel are unjustified. One cannot look at the alkaline value alone to determine the value of the water. While many of the best waters should be slightly alkaline, it has much more to do than pH alone. There are a plethora of testimonials and so-called scientific studies on the Internet claiming alkaline water will cure your every ill. Many consumers, struggling to make sense of the scientific jargon, eventually throw up their hands in frustration. The reality is, most of the circulating information is distributed by clever marketers, with very little scientific validity to back up their claims. Complicating matters is the fact that most water alkalizers are being marketed by multi-level marketing (MLM) companies with less-than-stellar ethics. They sell you a very expensive machine, for which you get a good discount if you sign up as a rep, and once you're part of the MLM, you can't very well change your mind about its benefits (especially if you're going to sell the units) - even if you realize that the alkaline water is no longer "working" for you. I have been personally approached many times and encouraged to sell these systems and there would have been large revenue streams had I chosen to do so, but I would never promote anything that I would never use personally, and I can assure you that I would never use most of the machines on the market that produce alkaline water as a regular source of water.

He goes on to say:

There are some scientific studies that really argue against alkalinity, at least with respect to preventing or treating cancer. Consider the research by Robert Gilles, who has studied tumor formation and acidity. According to Gilles, tumors, by their very nature, make themselves acidic - even in an alkaline cellular structure. In other words, they make their own acidity. Scientists who are in the process of developing prototypes for potential new anticancer agents that selectively kill tumor cells by interfering with the regulation of intracellular pH , have found that alkaline treatments do NOT have the desired effect - but strongly acidic treatments do. Talk about fighting fire with fire - they are fighting acid-loving cancer cells with acid! LESS alkalinity inside a cancer cell seems to be what you want, not more. So, all of those salesmen promising alkaline water will lower your cancer risk are completely clueless when it comes to what the scientific research actually shows. Even more interesting is a 2005 study by the National Cancer Institute , which revisits the use of vitamin C (ascorbic acid) to treat cancer. They found that, in pharmacologic doses administered intravenously, ascorbic acid successfully killed cancer cells without harming normal cells. This is yet another example of cancer cells being vulnerable to acidity, as opposed to alkalinity . It's clear that the relationship between alkalinity and cancer has been grossly oversimplified by those jumping to premature conclusions - and of course by those trying to profit off your fear. The bottom line is that alkaline water isn't cancer's magic bullet.

I personally know many people who have turned to alkaline water in their cancer treatments... sadly most were unsuccessful. Like everything in life, Balance is the Key:

As is true with many things, in the end it's a matter of balance. Water that is too acidic or too alkaline can be detrimental to human health and lead to nutritional disequilibrium. This was demonstrated in a Swedish well water study, which found both pH extremes to be problematic. Your body simply was not designed to drink highly alkaline water all the time. So I believe it's best to be VERY careful when it comes to something as foundational as the water you drink on a daily basis. If you get it wrong, you could really cause yourself some major damage. It makes sense that you are designed to drink water that occurs naturally, which excludes alkaline water with pH levels of 8 and above. And if you drink alkaline water all the time, you're going to raise the alkalinity of your stomach, which will buffer your stomach's acidity and impair your ability to digest food, as low stomach acid is one of the most common causes of ulcers. This can open the door for parasites in your small intestine, and your protein digestion may suffer. It also means you'll get less minerals and nutrients over time - in fact, some of these health effects can already be seen in hardcore alkaline water drinkers. Alkalinity is also potentially a problem because it is antibacterial, so it could potentially disrupt the balance of your body's beneficial bacteria.

Finally, Dr. Mercola writes this:

What you want is pure water - water that is clean, balanced, and healthful, neither too alkaline nor too acidic. Ideally, the pH of your water should be somewhere between 6 and 8. And some of the most healthful waters in the world - that which emerge from mountain springs - are actually acidic in the range of 6.5 and would absolutely be my preference if it were readily available. This mountain spring water is 'structured' in a way that is not well understood.

Cobalt Hyper-Safe Bacteria-Killing RO

Most of us do not have "mountain spring water" and even if we did, someone might contaminate it with something toxic. Happily, there is a solution. Reverse Osmosis removes the largest spectrum of contaminants of any water treatment process and the pH is generally in that 6.5 range. I am of the opinion that there is no healthier water on the planet that is readily available like RO water. It's not a prefect process or product, but it's closer to what your body needs than alkaline water.

Cheers!