Three days without water.
So now we've taken care of any medical issues and found a way to maintain our body temperature now we need water.
More or less we are made of 60 to 70% water and that balance has to be maintained to avoid dehydration. Making sure you have access to water either as a supply or methods of accessing and purifying, filtering or distillation are essential.
Dehydration also plays a part in how efficient your body maintains it's temperature either staying warm or keeping cool as well as the function of your body.
I've seen a reference that 2% loss of water in the body can impede judgment by 25% and the effects spiral down from there. That coupled with the fact that the majority of people are dehydrated as it is can put us in a predicament depending on the temperature sooner rather than later.