This paper is about dynamics of waters of the Keret Bay in the Chupa Estuary, Kandalaksha Bay of a semi-enclosed Arctic sea – White Sea, Russia. It is a small semi-isolated tidal estuary with special features because of mixing fresh waters of River Keret and Chupa Estuary during tidal cycles, and due to complicated topography of the area. Water exchange in tidal Keret estuary happen out through three small straits. Hydrographic surveys of the water area were carried out during summer seasons of 2006-2014. In 2009-2012 we used remote sensing of waters under surface by georadiolocation method, which allows to trace in situ a boundary of fresh and saline waters with high spatial and temporal resolution. Applying this method made it possible to delineate the area of fresh water propagation within the estuary at different stages of tidal cycles. We also used numeric hydrodynamic model of tidal phenomena in the Keret bay based on algorithms of famous hydrodynamic model POM (Princeton Ocean Model). Numerical experiments with various types of the open boundary conditions show that the model skill score of small tidal basins is fully determined by open boundary conditions. The use of the tidal sea level oscillation at open boundary condition gives insufficient results. The best skill score is achieved by setting the radiation open boundary condition while using current and sea level data. In this case, a comparison of the model results and sea level and current measurements shows the best agreements: the correlation coefficient between the modeled sea level and the measurements is up to 1, and the correlation coefficient between the simulated and observed currents equals to 0.9. The residual tidal phenomena were estimated by averaging the model results of sea level and currents, and it almost completely coincides with the mean sea level.