The potential of hydroacoustic methods widely used in coastal engineering geophysical survey is limited in the presence of gas-saturated bottom sediments. Under unfavourable conditions it is advisable to use electric prospecting methods. This article analyses the efficiency of modern electric resistivity tomographic (ERT) technologies of the surface and seabed observation systems for studying the geological sections in coastal water areas. Basic geoelectric and electric resistivity tomographic models are synthesized to assess the influence of water salinity and lithological composition of sediments on the results of marine electric prospecting. Petrophysical modelling data showed that, along with the influence of pore water mineralization on the ratio of specific electric resistivity values ​​of dispersed soils, the mineral composition of clay minerals has a significant effect. This effect is manifested as a shift in the position of the inversion point of specific electric resistivity values ​​of sandy-clayey soils with increasing cation exchange capacity ​​​​typical of different mineralogical types of clays. Results of numerical modelling of electric resistivity tomography sections using surface and seabed observation systems demonstrated that the seabed measurement system provides reliable information on geoelectric structure of sandy-clayey sediment sections, while the sections obtained from the ERT survey on water surface exhibit distortions in the geoelectric section structure and false anomalies. The advantage of seabed ERT for studying the geological structure of coastal marine areas was ascertained. Experimental work in the Luga Bay water area in the Gulf of Finland confirmed the efficiency of the seabed ERT for increasing the reliability of cross-well geological interpolation when constructing composite geological and geophysical sections. The use of seabed ERT ensured a continuous tracking of geoelectric boundaries corresponding to different lithological species in seabed sandy-clayey sediments.