Water isn't necessarily defined as life. Although to some extent, water has proven to be vital in almost all known living organisms in this life. Thus, water is always paired up with life to the point of as we all hear or read, water is life.
Scientifically speaking, water is essential in all forms of beings. Look it up on the net and everywhere it says that water is essential in promoting life. From the composition of the bodies of the organisms, to requirements in achieving cycles, to regulation of temperatures, and ultimately in sustaining life itself.
This also has been adapted to other studies as well. In psychology, more specifically in the interpretation of dreams, water generally symbolizes life or giving birth. Though this has changed over the course of time, there isn't that much debate over the matter that water definitely is life.