Water is essential for survival; dehydration can kill a person after only three days, while you can survive without food for weeks, albeit it would not be a very nice experience.
But as important as it is to get water, it is also essential to choose fresh, usable water, and if you have the chance to choose, always go with water that is moving instead of drinking or using pond or still water.
The faster the water moves, the better; that will generally ensure you that the liquid remains free of insects and their larvae, as well as a good degree of oxygenation.
Related:
Comments, tips and advice on this topic
Survival - Products and Services
Water and survival.