What happens to the body when it does not get the water it needs?
A human's body water is divided into functional compartments. One is the Intercellular Compartment (inside of cells), and one is the Extracellular Compartment (outside of cells). When your body does not get enough water, then the EC suffers first and so the EC organelles and tissue becomes dehydrated. Since most cellular membranes are semi-permeable, they will allow water to be moved from inside the cell to outside the cell, which means that the cell itself becomes dehydrated, as well.
Think of the cell like a grape. When it is healthy, it is nice a plump, but when it is unhealthy and dehydrated, it looks shriveled. The shriveling changes how the membrane reacts with the environment and causes the cells to be less efficient.
Water (H2O) also has powerful reduction and oxidation purposes in the body, so your many functional systems (e.g. nervous, muscular, lymphatic) suffer from lack of a chemical that is required for many biochemical processes. A prime example is the electron transport chain. The final electron recipient on the electron transport chain (the main source of energy in metabolism) is water. If water does not exist, then energy production is halted.
Symptoms of dehydration are fatigue, headaches, heat exhaustion, and others.