I suggest that you create a threshold at which you’re prepared to worry about the amount of money at stake. For example, my threshold is about $10. If an item costs less than $10 or my savings would be less than $10, I just buy or don’t buy something. If the money at stake is greater than $10, then I think it through.
In this case, I wouldn’t think twice about buying a bottle of water, because it costs less than $10.
You’d probably want to put a daily limit on how many of these discretionary purchases which you make per day, so you blow your entire budget in $9.99 purchases in one afternoon.