The marine resistivity tomography method can be useful for coastal engineering applications through the use of immersion cables, laid on the water bottom, thus achieving a deeper and more detailed detection of the formations underneath. Careful positioning of the profiles related to the available geologic, geomorphological, bathymetric, and drilling data and to the requirements of engineers, can give thicknesses of loose material as well as the bedrock relief. The formations and their interfaces are clearly defined either by calibration with available drilling data or by high resistivity changes which indicate discontinuities with resistivity values related to permeability and saline water presence. Two case studies are under examination. The first concerns the preservation of an old iron bridge in Lavrion Port with reinforcement and new foundations. Here, the geophysical investigation detects the formation on which the bridge was founded, as well as the loose material above it. The second case study concerns geophysical investigation asked by a construction company for the marine excavation work needed in the new Mesta port. Here, the geophysical measurements with the detected formations give valuable information with respect to the volume of material, equipment, time and cost of excavation as well as where the foundations will be laid for new constructions.