To exist, life must have a space in which to exist.
Water is that space.
All life, at least on this planet, is water based. The atoms and molecules which conduct the life process react with each other in water as the solvent. It is hard to imagine life without water. However, life might be possible in the presence of another solvent with qualities equal or superior to water. It may be that water is the solvent used by life on earth simply because it is here in much greater quantities than any other solvent.