Researchers in Sweden may have figured out why some whiskeys taste better with a little water mixed in. And it's not what most people think. It's because of a compound called guaiacol, which is what gives them that smoky flavor. (Pronounced GWHY-uh-call.)
Since water and whiskey don't really mix on a molecular level, they used computer models to see how the molecules interact. When you add water, it pushes the guaiacol to the TOP of the glass. That gives more flavor in each sip.
But they do say to err on the side of caution, because too much water could dilute it and kill the flavor. How much water to add. It depends on the type of whiskey, and also just personal preference.
From CNN.com - Why whiskey tastes better with a little water