H2O is necessary, but not sufficient.
It takes much, much more to sustain life as we know it than just liquid water. Water seems to be necessary, a good indicator, but is basically a catalyst for many, many other processes. Many other elements are needed, a relatively moderate temperature range, and resources which can be manipulated to provide energy. Radiation must be moderated somehow.
That's for life as we know it. The Moon doesn't seem to have liquid water and the radiation isn't moderated on the surface. Would be tough to find life there which isn't an artifact of Earthly activities.
Mars is different. Entirely possible to meet all the needs for life.