Researchers in Sweden think they've figured out why some whiskeys taste better with a little water mixed in. And it's not because it dilutes it. It's because of a compound called guaiacol that gives them that smoky flavor. (It's pronounced GWHY-uh-call.) Water and whiskey don't really mix on a molecular level. So they used computer models to see how the molecules interact. And when you add water, it pushes the guaiacol to the TOP of the glass. So you get more flavor in each sip. They haven't figured out exactly how much water to add. It depends on the type of whiskey, and also just personal preference. But they do say to err on the side of caution, because too much water could dilute it and kill the flavor.
http://www.cnn.com/2017/08/17/health/whiskey-water-taste-study/index.html