There are indeed a few locations in each park that cannot give water, because they have no Spigot for it.
Basically, the rule of thumb is if they serve any non-carbonated fountain drink, they can should be able to give cups of water. If they only have carbonated fountain drinks, they cannot give water.
I am not sure which locations this applies to at WDW off the top of my head, but I am sure there are some.
Also, if they don't serve fountain drinks, they cannot give cups of ice or water, as they have no cups.
And I know this should go without saying, but do not use the ice from the carts or ask to use it. It is unsanitary, as people will have been touching it throughout the day.
Also, you do not have to purchase anything to get the free cups of water. Another tip is always ask for a large cup of water. Some CMs will give the largest cups, while others will give the small Soda size if you do this and most will give the child cups if you don't specify.
Another place where free water is limited is anywhere that Sodas are self serve in the parks. I believe this only applies to the Electric Umbrella, but just a heads up.