Don't waste money or dining credits on bottled water in the parks, you can get free cups of water at any quick service restaurant.
Go up to any counter service restaurant in the parks and ask for a free cup of tap water. You don't have to get food with your water, but if you do get a meal, make sure you get that extra cup of water too.
There are water fountains all around the park too, so bring a water bottle with you and top it up as you go. Generally water comes out of the fountain on the warmer side, but it's better than dehydration.
Some Disney hotels leave a free bottle of water each day in your room so this is great for topping up in the parks. You can also order packs of water from an outside supplier, such as Garden Grocer or Amazon. This works out much better value than buying a $3 bottle from a park concession stand.