Are you renting a car? If you are, there alot of great options for finding water, breakfast foods, snacks, and other items that are a pain in the butt to fly with or Disney charges way to much for(BEER). Costco, Publix, Goodings, Super
Walmart are all within a reasonable driving distance.
A one day car rental can be as low as $25 + $5 for the water = $30
24 bottles of water at WDW x $2.50 = $60
If you aren't renting a car, I believe Costco, Staples, Wegoshop, gardengrocier, all will deliever water and other items but there maybe service charge from them and the resort or a minimum $ amount to order.
I always rent a car and stop on my way from the Airport. I'm actually making my list for our trip in 9 days right now.