Typically once you have established warmth, water and staying hydrated becomes your next issue. Under normal conditions a human will drink close to a gallon of liquid during a day. If you are at altitude, or in a arid region like a desert this need may greatly increase. Part of this equation is genetic. I know some people who will go through a gallon of fluid in four hours on a hike, and others who can go all day on less than two quarts.
If you have a supply of water you should not ration it. There is a common myth that you should stretch your supply as long as possible. Rationing water only deteriorates your condition which will increase the chance of dehydration and the complications that dehydration causes. If your water supply runs out, then limited movement to find water is necessary. If you have gone more than eight hours with any water and none is in your vicinity, it is time to move on or find alternative sources.
In the damper regions of the southeastern United States, water can be found sometimes by digging just a short distance under the ground. In desert regions, you can use your trash bag in your survival kit to make a solar still. Digging a hole, line the bottom with part of the plastic bag and then over the top of the hole secure another section of plastic. Use stones to anchor this down and put a stone in the center of the top sheet so the bag hangs down slightly into the hole. Moisture will collect during the day and provide a small amount of water. You can also use your plastic bag to capture rain if you are lucky.
Drinking out of rivers, lakes and streams pose interesting problems. Just because water is clear does not mean it is safe to drink, and just because it is murky does not mean it is not safe to drink. In places like Badlands NP in South Dakota, none of the water present in the park is safe to drink due to natural contamination. Bacteria and other parasites can await in water taken from rivers and streams. Fast running water in a rocky and sandy bottom is typically a safer bet, but not a 100% one. If you do not have water purification tables as part of your survival kit, you can boil water over your fire. You can use almost any container, even a small soup can if necessary. If all else fails in the short term you are better off drinking the water.
Dehydration is more insidious then hypothermia. Dehydration tends to creep up on a person and we tend to ignore the signs due to the routine of our daily lives. To check for dehydration grab a pinch of skin on the back of your hand, hold it for a few seconds and let go. If the skin does not go back, but remains pinched, you are dehydrated. Cottony mouth, dry eyes, and a lack of ability to sweat are also signs. In severe dehydration, dementia can take over causing poor judgment.
If you get dehydrated there are some definite don'ts you should follow. You should not drink blood, urine or other fluids in attempt to quench your thirst. Urine and blood both will only make you more thirsty. You should not force yourself to move, especially during the day. Sweating and even labored breathing accelerates dehydration. Stay out of the sun and stay away from salty foods (which typical survival rations are). You can go much longer without eating then drinking. A lack of water is a reason to violate the second rule of hugging a tree and to move on. You should make this decision before dehydration sets in. When moving without water take your time and move at a conservative pace, try to move downhill or on flat terrain.
The biggest factor to remember is that you can go three to seven days without any water at all. If you have followed the second rule of hugging a tree, notified a proper authority of your route prior to leaving, help should be on the way before this three day window. Most rescues of individuals happens within the first 24 hours.
A roof over your head, finding shelter...