  In this paper we investigate the influence of ultra thin water layer (1-1.5
nm) on the van der Waals/Casimir force between gold surfaces. Adsorbed water is
inevitably present on gold surfaces at ambient conditions as jump-up-to contact
during adhesion experiments demonstrate. Calculations based on the Lifshitz
theory give very good agreement with the experiment in absence of any water
layer for surface separations d>10 nm. However, a layer of thickness h<1.5 nm
is allowed by the error margin in force measurements. At shorter separations,
d<10 nm, the water layer can have a strong influence as calculations show for
flat surfaces. Nonetheless, in reality the influence of surface roughness must
also be considered, and it can overshadow any water layer influence at
separations comparable to the total sphere-plate rms roughness w_{shp}+w.
