Alcohol and water

I’m 19 years old and can legally drink where I live. My parents left a bottle of whiskey at the house a while ago and told me not to touch it, it was just opened and only a small amount was used. I ended up having some and I can see there’s a small difference, enough to notice if you really pay attention. NOW, can I add some water in? Only about a shot worth. It really isn’t worth going and buying a whole other bottle just to top it up a little. I’m also afraid of going into the liquor store, though i’ve done it a few times. I know people added water to clear alcohol like vodka etc. But i’m not sure how it will react with whiskey.