The effective barrier height for electron tunneling is measured across the aqueous capacitor junction composed of a scanning tunneling microscopy (STM) tip and a surface. At a junction distance of a water monolayer, the barrier height strongly varies with the polarity and the magnitude of the applied junction voltage. This monolayer barrier height increases with an increasing positive value of the sample bias, while it does not for the negative bias. Such asymmetric variation of the tunneling barrier height manifests the influence of water molecular geometry on electron tunneling.