You might not think about the water you use because it’s always there. This isn’t always the case for large parts of the world’s population who live in arid areas. They cannot get water easily because they live in deserts. As well as not being able to access water at all, there is an even larger part of the developing world who cannot get clean water because all of their local water sources are contaminated. Some of the contaminants might be biological, such as diseases that are easily transmitted, the contamination might always have been there because the area was only developed recently, or the contaminants might be human in origin, such as if a local mine has polluted a waterway. The point is, this is not just a hypothetical question because large parts of the world do literally go a day without water.
The average person in the USA uses 80 to 100 gallons of water per day (http://water.usgs.gov/edu/qa-home-percapita.html) and not much of that is used for drinking. If you add up all of the water you use for cooking, washing, bathing and cleaning in addition to what you drink, it’s a lot of water.
There is no shortage of water in most parts of the USA, so it’s not normally a concern there. In the developing world, water shortages are a serious problem.
The average person is 65 % water, so we need to consume a lot of water throughout the day to stay hydrated. You don’t want to be uncomfortable, so you have a sip every couple of hours. If you don’t have water for three days, you will probably die. There are some people who have survived 10 days without drinking water, but they are exceptions and they were not in dry climates. If you are in a desert it might even be less time that that.
So we might survives our day without water, but it would mean we can’t clean, bathe or wash using water. This would be an unpleasant, but survivable day.