Why is water bad?
Water relaxes some of the weak internal bonds in hair (called hydrogen bonds) and causes the hair to lose its shape. So using a water containing hairspray can cause you to lose style definition almost instantly.
You WANT water in some products (e.g., creams and gels) because it helps you work the product through your hair to get the style you want. But if you’ve already got your style set and you just want to lock it in place – then you do NOT want water to touch it because it will cause the style to droop.
And don’t worry about spraying alcohol on hair. I’ve never seen any proof that alcohol in hairspray really dries it out – the alcohol is in very small droplets and it evaporates very quickly.
When is water good?
Why, you might ask, would you EVER want water in a hairspray?
Some people like water in their hairspray because they can use it as a “working” spray or “shaping” spray to help style their hair. This is especially true for non-aerosol (pump) hairsprays because they HAVE to contain water. (That’s because countries like the US limit how much alcohol a product can contain because it’s a Volatile Organic Compound (aka VOC) that can contribute to air pollution.)