The data of direct measurements of hydrological conditions in the pre-flood part of the Vistula lagoon of the Baltic Sea at different phases of water exchange in the presence of ice cover and polynya were presented. The data of the expedition work in the winter of 2021 were used in comparison with similar data of the expedition work on the study of sea currents in the summer. It was shown that the polynya was formed in the zone of the inflow of warmer sea water, and, thereby, can serve as an indicator of the depth of penetration of full-volume sea water intrusions into the lagoon area, realized in the specific conditions of the narrowness of the strait and the presence of an underwater bar.