Using first-principles gradient-corrected density functional theory, we have studied the adsorption of H<sub>2</sub>O molecule on a single-wall carbon nanotube and found that H<sub>2</sub>O 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 C<sub>60</sub> molecule on its electron transport property and ofund that encaging of C<sub>60</sub> 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 C<sub>60</sub> encaging is explained on the basis of charge transfer between the host nanotube and guest molecules.