The first living organisms were bacteria that thrived in water.So, they needed water to survive. Since then,DNA,RNA,and the other things that we need to survive have come from those.

Water has some unique solvent properties, which make it absolutely vital in the chemical processes which power life. It stores a fair amount of heat energy in the bonds between its own atoms, as well. Water, as you probably know, is two atoms of hydrogen sharing electrons with one atom of oxygen. What makes this arrangement important is that it is not equal sharing; the oxygen atom tends to pull the electrons away from the hydrogen, so that the electrons spend more time hanging around the oxygen. Electrons carry a negative electrical charge, remember -- so this arrangement means that, even though the electrical charges of all the atoms together are balanced overall, in reality the oxygen end of the molecule tends to be negatively charged, and the hydrogens tend to have a slight positive charge. And THIS means that water is attracted to any other molecule which has a slight electrical charge, and will make multiple, small, transient bonds with those molecules. This simple fact is what allows a lot of chemical reactions to proceed, which power metabolism -- the transfer of energy from one molecule to another in order to power mechanical work, with the system returning to a resting state at the end. Water makes the chemistry possible _because_ it has the ability to make lots of easily-formed and easily-broken electrical bonds with other molecules. And there are no other liquids which behave in quite the same manner. Gases would have been no use, because they are too diffuse for energy-reactions to take place on a useful timescale for powering life, and solids are no use because the molecules in solids are too rigidly held to participate in the kind energy transfer reactions that life uses. That's what is at the base of it.

Actually the answer is ... If this liquid water had not accumulated, membranes could not have formed, because they take on their bilayer structure in water. No membrane, no cell. (Hi vet tech student! ;)