The interfacial region of aqueous systems also known as the electrical double layer can be characterized on the molecular level with second harmonic and sum-frequency generation (SHG/SFG). SHG and SFG are surface specific methods for isotropic liquids. Here, we model the SHG/SFG intensity in reflection, transmission, and scattering geometry taking into account the spatial variation of all fields. We show that, in the presence of a surface electrostatic field, interference effects, which originate from oriented water molecules on a length scale over which the potential decays, can strongly modify the probing depth as well as the expected intensity at ionic strengths <10(-3) M. For reflection experiments this interference phenomenon leads to a significant reduction of the SHG/SFG intensity. Transmission mode experiments from aqueous interfaces are hardly influenced. For SHG/SFG scattering experiments the same interference leads to an increase in intensity and to modified scattering patterns. The predicted scattering patterns are verified experimentally.