Water is the single most important requirement for a healthy human life, you could live several days and even months with no food, but you would die in a day or two at most without water.
So it is especially important not just to consume water but to consume enough water and stay well hydrated, not just to stay alive but to stay healthy and keep a good appearance.
First of all, you should know that ingesting at least 2 liters of water every day is just a rough estimate, and that amount actually varies wildly from one person to other, as well as the sources of water for every person: while one person may get most of its water from foods such as soups or fruits and even desserts, others may get most of theirs from drinks (it has been estimated that as much as 20% of our daily requirement of water is provided by solid foods).
A recent report has been compiled and published by the United States National Research Council; it is the working arm of the United States National Academies (which groups the National Academy of Sciences, the National Academy of Engineering and the Institute of Medicine) and its job is to write and publish reports such as the one mentioned, with the aim of informing the public in subjects of science, medicine and engineering.
This report included a recommendation for the ideal amounts of water every person must ingest daily: 3.7 liters for men and 2.7 liters for women; however, you should be aware that people living in warmer climates (especially warmer and humid climates such as dessert areas) and those who are physically very active will obviously need to consume a higher amount of water in order to compensate for their higher amount of sweating; other persons who should increase their intake of water are those on a high-protein diet, those on a high-fiber diet, pregnant or breastfeeding women and people who are sick (especially those with gastrointestinal diseases of any type).
That amount is total and it does not matter if it is taken from liquids and fluids or from solid foods: remember that some solid foods have a pretty high amount of water, especially fruits and vegetables. It is interesting to compare these recommendations to the older recommendations published in the first report of 1945 by the same National Research Council, which stated that the ideal amount of water every person had to consume was of 1 milliliter of water per every calorie of food. Since the average amount of calories ingested by a healthy American adult was of 2,000 to 2,500 per day, then the amount of water recommended was of 2,000 to 2,500; a figure that helped spread the myth of 2 liters of water per day as the ideal amount of water to be consumed.
And, speaking of myths, another well-known one states that the ideal amount of water to be taken is of 8 glasses of water per day, and while the best way to be hydrated is by drinking water and 8 glasses might provide the 3.7 liters (or 2.7) of water you need every day, at least a 20% is already provided by solid food (as mentioned above), so it is not necessary to drink 3.7 or 2.7 liters of water every day, just to drink what food has not provided.
Institute of Medicine. (2005). Dietary Reference Intakes for Water, Potassium Sodium, Chloride and Sulfate. The National Academy of Sciences, Institute of Medicine. Washington, D.C.: The National Academies Press.
Institute of Medicine. (2006). Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. The National Academy of Sciences, Institute of Medicine. Washington, DC: The National Academies Press.