On twelve testing fields located in the main water basins of the Gda?sk and Szczecin harbours, the efficiency of a new depth technique was verified. The technique that determines a safe nautical depth is defined by a density criterion of a mud-water medium. The measuring results of the medium density varied greatly depending on a physical phenomenon applied in the measuring equipment. The differences found are mainly attributed to a different gas content in the surface mud sediments as well as to reological properties of the sediments. The results of the numerical simulation of acoustic waves propagation in the mud sediments clarify a depth overestimating by the echo sounder of a low frequency.