Lanefan
Victoria Rules
The first three each could be quite realistic, depending on circumstance and context of course. I'm a little dubious about the sombreroed stilt-wearing dudes, however; though if on finishing the pie they then ate the winter wolf we might be back on track.To carry it a bit further --- let's suppose that there's far more options available to "forward" the fiction than even that.
- The orcs immediately band together and go on a raiding rampage to a nearby village.
- The orcs are okay with it, because they didn't really like the taste of that water anyway, and can sustain healthful hydration from the demonic fountain that's spouting blood on Level 19 of the dungeon.
- The orcs are convinced that it was a sign from Gruumsh, and they should immediately evacuate the area.
- The orcs laugh and smile and eat apple pies together with their pet winter wolf, while wearing sombreros and stilts because clearly these orcs are acrobats.
Now of course the argument here might be, "Well none of those are realistic! None of those follow-up results seem to follow from the authored fictional cause!"

Yes she is; and in this case she should be, as it's both her place and her job to determine* how the various elements of the game world (in this case, some orcs) react to actions performed by the PCs (in this case, cutting their water off). When and if they become observable by the PCs it's also her job to narrate these reactions.And this is true---but it doesn't change the fact that no matter what result is chosen by the GM, he or she is still the one authoring the fiction.
* - by whatever means she likes - could be pre-authored, could be dice rolls, could be off the top of her head, some combination of these, or whatever.
Lanefan