The tip of an oscillating cantilever experiences a strong variation of energy dissipation with the distance just before contacting the surface. This allows scanning force microscopy (SFM) imaging in non-contact mode with high resolution. The potential of this mode is demonstrated by the study of an extremely soft and easily detachable sample such as water adsorbed on graphite, gold and mica substrates. Flat layers of water, ranging in height from molecular dimensions to several bilayers, are observed on the different surfaces studied.