Did you double check the kitchen to see if the bottle was there (on one of the northern tables, IIRC)? Once you have the correct bottle in your inventory, there should be a speech option to give it to the prisoner.
If that doesn't help, I have updated the
Inventory files topic with the water bottle.
I think the programmers expected most people to either loot the kitchen and make it to the prisoner without drinking that bottle, or to reach the prisoner and then go looking for water to bring back right away.
Most quest items are unique in some way (though not all unique items are quest related). Actually, I can not think of another quest item that is basically a duplicate of a common item found elsewhere in the game (the quest water bottle does look different, but not so you would think it was special). The optional BF quests can be to find equipment similar to that which is randomly generated, but the location indicates it is unique (BF quest items and monsters are always on the third level of a BF dungeon).
