Following the completion of the Eurasian Tunnel in 2016, the Municipal Government of Istanbul plans to construct a second subsea tunnel across the Bosphorus Waterway between the European and Asian sectors of the city. We conducted an offshore 2-D sparker seismic survey in the Bosphorus Waterway to delineate geometry of the sub-bottom sedimentary sequence and geometry of the sediment-bedrock interface. We constructed a geological model based on the interpretation of the seismic image associated with the center line along the planned tunnel traverse. This geological model was subsequently used to determine the locations and depths of the geotechnical boreholes. Formation tops determined from geotechnical borings confirm the geological cross-section constructed from the seismic image in depth along the tunnel traverse, except within a segment on the European side where the strong currents of the Bosphorus have severely distorted the 2-D line traverse. This investigation demonstrates the compelling need for a seismic survey prior to geotechnical drilling for accurate description of the soil column both in the lateral and vertical directions within the project area. The seismic survey provided a cost-effective planning of geotechnical drilling to determine the number of boreholes, and where and how deep to drill each borehole. Additionally, the geological model constructed from the seismic image was used for the preliminary tunnel design. Nevertheless, a 3-D survey will be required prior to the final tunnel design and construction in order to resolve the uncertainty of the slope boundary of the top-Trakya bedrock on the European side.