Among other functions, water enables cells to transport chemicals, helps their temperature remain more stable and keeps the cells turgid. Every living cell on the planet requires water to survive, which is part of the reason scientists suspect that extraterrestrial life is most likely to occur on planets or moons with liquid water. Because water is used to flush toxins from the cell, it must be replenished frequently.
Because water is a polar molecule, it is an excellent solvent, and it can carry a wide variety of dissolved substances in it. This not only helps cells to transport dissolved substances within the cell, but also it helps them to absorb nutrients and expel wastes as well. When a cell is not properly hydrated, such transport functions slow, causing problems for the cell.
Because water has a relatively high specific heat value, it takes a lot of energy to heat up (and cool off), which means that cells do not experience drastic fluctuations in temperature. Stable temperatures help cells to perform more efficiently, as the functions of the cell often slow when the temperature falls and speed up when the temperature rises. Additionally, the water inside a cell helps the cell to remain somewhat pliable, while still retaining its shape.