Onshore applications of direct current resistivity (DCR) along shorelines suffer a short-circuit-like phenomenon due to electrical current flowing through a more conductive body of water rather than ground. Our study of the numerical simulation of DCR data with a three-dimensional forward model demonstrated that the apparent resistivity was reduced as a function of the sea depth and the distance of measurement site to the shoreline. Furthermore, it was concluded that the "marine effects" on DCR data (i.e. reduction in apparent resistivity) become nonnegligible as the ratio of half-electrode expansion (AB/2) to the distance to the shoreline is larger than one. The reduction in apparent resistivity reaches its highest levels as the ratio approaches ten. Our survey conducted along the coastal line of Northwest Turkey clearly showed that if the "marine effects" are left untreated, one- or two-dimensional inversion yields incorrect resistivities for underlying units and therefore undermines the credibility of survey results. In the paper suggestions are made to handle such situations.