Dehydration is a serious condition, it can quickly come upon us, especially during warm/hot environmental conditions or when engaged in heavy labor, including hiking and even swimming.

However, if the weather is 55F or less (maybe even 60F or less), wet and/or windy or very cold, hypothermia presents a significantly greater short-term dangerous and potentially lethal situation, then dehydration. Gear in the terms of vapor/wind blocking, appropriate clothing and a ready source of high-energy/calories is actually more important then water.

Just my humble opinion-
Pete