You can...I'm not sure if you are talking about bringing a case of bottled water or if you are talking about bringing something to carry a drink in.
The ship's water is very good...I don't feel the need for bottled water. On excursions, it is helpful to carry something to drink. On deck 9, drinks (water, milk, lemonade, fruit punch, coffee, tea, hot chocolate, iced tea) are available all the time.