Trapped people in fiction are more afraid of hunger than dehydration, unless it's hot.
In real life, if you get trapped in your basement your biggest motivation for escape would be that your going to die of dehydration in less then a week. If that same thing happens to a fictional character, expect every one involved to talk about how the person involved was "left down there to starve to death". The only time you'll ever hear about people in fiction fearing dehydration is if they're trapped somewhere hot, like a desert. This is presumably the situation because people associate dehydration with being hot and writers assume viewers are too dumb to realise a person will eventually dehydrate to death at any temperature if denied water indefinitely.