Personally i rarely bother. The main reason being laziness. I find bottled water, bottled in plastic, tastes overpoweringly of plastic and wouldn't want to put that into a fine whisky. If you should add water i'd go for a glass bottled water, or tap water.
Someone with more experience about amounts of water will be along shortly - maybe.
I figure the master distiller put a lot of care into making this whisky taste just right. Why would I want to mess that up? If they'd wanted more water in it, they would have added it themselves, right? What is it, whisky concentrate?
I have tried adding a few drops, on occasion, to see if it would make a difference. Every time, I've either not detected a difference, or it had a negative effect. I've always used either tap water or run-of-the-mill (bottling plant?) bottled water. Be careful of tap water - some of it can have quite strong flavors and aromas. My town's water is heavily chlorinated, for example.
Adding water is strictly for evaluation of the whisky. Nosing high proof liquor deadens the smell receptors in your nose rendering them much less effective at isolating individual aroma characteristics.
For enjoyment do whatever you want, add ice/water/coke/juice/puppies it is irrelevant. You should drink whatever tastes best to you.
IMHO there should be no reason to really add water to anything bottled below 46% as typically those have already been watered down by the distillery itself. (Rare exceptions of those extremely alcohol greedy casks or very old casks which can sometimes naturally fall at or below that point).
But that being said when you get to Cask Strength products they can occasionally be incredibly overpowering. That being said they are often more enjoyable when enjoyed naturally, but sometimes when getting to know them it can be much more enjoyable to dilute them a bit. That being said I now have little issue drinking stuff at 55% or so, but one thing I have at 60% can still be incredibly overwhelming, though exceedingly delicious.
If I do add water, I add tap water, that I have tried to let rest for awhile both to come up to room temp and loose some of the chlorine.
Adding water is strictly for evaluation of the whisky.
Well that's kinda what I've assumed. They say that water "opens up the flavors and aromas". I've never been certain what that meant, but guessed that adding water dilutes the whisky enough, that the individual elements are easier to discern.
Nosing high proof liquor deadens the smell receptors in your nose rendering them much less effective at isolating individual aroma characteristics.