Water is one essential your cat absolutely can't do without. She'll need to drink at regular intervals throughout your trip, just as she would at home. She may need water even more often if you're going to be spending time in a warm climate.
Though tap water is fine for your cat at home, drinking from a tap in another city or country could be hard on her tummy. Either spend the extra money and buy bottled water, or bottle it yourself from your home faucet before you leave.
Make sure your cat always has access to water while you are traveling. Either attach a bowl to her carrier, or make frequent rest stops so she can have a drink.