One of the primary needs in any survival situation, winter or summer, is drinking water. Having enough water is a very real issue, and it's important to understand that you must prioritize how you use water in a survival situation. You can live without bathing, without washing your clothes, and without flushing the toilet, but you can't live without water to drink.
So how much is enough? That all depends on the ambient temperature, humidity, elevation, your state of health, and your level of activity. During hot weather, if you can manage to stay reasonably cool by using a shelter to avoid direct exposure to the sun and hot wind, you reduce the amount of water intake you need. During all seasons, if you pace yourself so you don't lose body moisture to perspiration, you reduce the amount of water intake you need. If you limit your food intake, you can get along on a little less, but not much. If you are ill, you might need more than if you are in perfect health. At high elevation, the dry air will dehydrate you faster than at lower elevation. The same goes for extremely dry atmospheric condition in the desert vs. the more humid environment of a lowland forest.
The safe thing is to plan for a gallon per day per person for drinking and limited cooking. Yes, you can survive on less, but it's good to plan for more than to plan at the bare minimum. Note that a gallon of water weights about 8.5 pounds, so you're not going to be able to pack around enough water to last very many days, unless you have a vehicle to carry it. If you can safely shelter in place and have a sufficient water supply that you wisely stored in advance as part of your emergency preparedness plan, that is the very best scenario.
If you are forced to evacuate in a vehicle, take as much drinking water as you can load, along with the rest of your 72-hour kit.
If you are forced to evacuate on foot, you'll be limited to a couple of gallons of water, plus your packable 72-hour kit. This will require you to be able to collect and purify any water you can find, so carry a good water filter and some sort of canteen or water bladder or other container for this purpose.
Under normal conditions, you can survive for a few days without water. That doesn't mean you'll be happy about it and full of vigor, but you won't die from dehydration for at least a couple days. If you limit activity and remain sheltered, you can last a lot longer. Some of the survivor of the recent catastrophe in Haiti were trapped in rubble, unable to drink, for nearly two weeks. Some survived longer than that, with a very limited supply of water.
Sickness and injury place a heavy physiological burden on the body, and more water is needed during periods of physical trauma. Likewise for nursing mothers — they need more.
When setting up your personal or family emergency preparedness program, plan for one gallon of pure water per day per person. Organize your supply in containers that are easy to carry (a 55-gallon drum full of water weighs close to 500 pounds and is impossible to move). Be able to grab water bottles and stuff them in small places in your pack or vehicle. Make sure you have a good quality water filter, and know how to use it. Water purification tablets are good, too, if everyone in the family can tolerate them without adverse reaction (check with your doctor about this).
During evacuation, if you have the luxury of choice, choose to travel where you can obtain fresh water along the way to purify. In other words, make your contingency plans now to know what possible routes you might take if you have to get out of the area, and make sure there are some sources of water along the route.