There's really no need to buy disposable bottles of water...it's so wasteful and expensive.
A lot of folks don't like the taste of Florida water (myself included) so you can buy one of the Brita water bottles that have a filter in them and refill your bottle as needed at your resort, water fountain, or at counter service locations where you can ask for a free cup of water. If you don't want to carry a bottle around, all counter service locations will give you a free cup of ice water if you ask, and there are also water fountains located throughout the parks.
If you're really set on having individual bottles of water, many of the resort gift shops sell cases of water for about $6.50 (approximately) so there's no need to go through
Amazon, Staples, etc to ship water to your resort.