Using first-principles gradient-corrected density functional theory, we have studied the adsorption of H2O molecule on a single-wall carbon nanotube and found that H2O molecules adsorbed on the nanotube surface with hydrogen forming a weak bond with the surface carbon atom. Subsequently, Green's function based Landauer-Buettiker multichannel formalism is used within tight-binding model to calculate the electron transport. Our calculations suggest that the conductivity of the nanotube is reduced with water adsorption is consistent with recent experimental measurements. We have also investigated the effect of endohedral doping in nanotube with C60 molecule on its electron transport property and ofund that encaging of C60 molecule in a semiconducting nanotube enhances the conductivity of the tube. We have compared our conductance results with available experimental results. The decrease of electronic conduction due to water adsorption and the increase in conductivity due to C60 encaging is explained on the basis of charge transfer between the host nanotube and guest molecules.