Whiskey and Water

According to whisky lore, the best water to use is the same natural, pure spring water used in the production of the whisky itself (every distillery has its own natural water source). Unless you live near the distillery (or buy ridiculously-priced local spring waters sold by some distilleries), this is impractical. Your best bet is to use bottled natural spring water, or a mild bottled mineral water (taste it first, it should taste clean and clear and not like chemicals). If your tap water is drinkable and not especially hard or soft, or if you use a filtration pitcher, you can use this water in a pinch too. Lastly, this water should be ROOM TEMPERATURE or just slightly cool. Cold water will do the same injustice to the whisky that ice does, and dull its

http://scotchnoob.com/2011/03/09/water-ice-or-neat/

I too had never heard of this until I read @user5561's answer. So although it's news to me, I'm going to venture an answer. I believe the "reasoning" is as follows:

The overall taste of the whisky will be comprised of the flavour of the water, the flavour compounds generated by the fermentation and distillation, and finally the flavour generated by aging the whisky. Therefore, if you wish to water the whisky down, you should use the same water the distillery used so that you are not adding additional flavour components to the final drink. If you use a different water, you will be introducing flavours that weren't present in the whisky as it came out of the bottle.

Now there are three assumptions this belief will be predicated on:

1. There is a human being on this planet with a sense of smell so developed that they would notice the difference.
2. The water you are using will somehow always make the drink taste worse and never complement what's already there.
3. The original flavour components in the water the distillery used have not undergone any changes during the fermentation, distillation and aging.

I take issue with the first two assumptions. You won't be able to tell the difference unless you're using water you just scooped out of a swamp and, with good water, there's no reason to believe it could not possibly improve the drink. I doubt the third but don't have any evidence either way.

I've come across a similar argument used for pizza, coffee, stock, etc... For example, since coffee is mostly water it's the most important ingredient. It seems logical until you realize that the neutral flavour of water is so subtle that it's quickly overwhelmed by just about anything you add to it (especially ground coffee beans).

When making whisky, water is used at several stages; first, it's used in the vat to give the yeast more mobility to fully digest all the starches (and to ensure the alcohol concentration stays low enough to not inhibit the yeast's growth). That water is removed during distillation (which typically produces a spirit between 140 and 150 proof; a traditional open-air still simply cannot remove all the water from the alcohol), and then a little fresh water is added back in, producing a "cask strength" somewhere in the 100-120 range. The extra water slows evaporation of the alcohol and keeps the barrels hydrated during the aging; regardless, some of the spirit is inevitably lost to the air (the angels' share) and to the wood of the cask itself (the devil's cut) as the barrel sits in the aging vault. Finally, just before bottling, the cask's proof is tested and the whisky diluted to 80-100 proof for sale in the U.S.; a few distillers also market a "cask strength" that doesn't get this last step.

Therefore, the whisky in the bottle contains a good dose of whatever's in the water in the brook flowing by the distillery, typically referred to as "peat" as most of the water in that region filters through layers of peat moss before arriving at a stream (and over in the Islay reagion, it's less stream, more peat bog, leading to the quite extreme flavor of whiskys like the Laphroiag).

It's traditional to add a splash more of water to a whisky to "open it up"; the whisky loses some of the powerful alcohol sting and displays more of the complexities of its taste (two primary variables in Scotch are a smoky note from the char of the barrel and the earthy peat from the water). Filtered water is generally acceptable, especially across the pond where local water from the distillery is hard to come by, but hardcore Scotch connoisseurs who look for those earthy peat tones unique to the spirit will prefer to add more of that same flavor in the water.