The method based on the numerical modeling of the processes, forecasting and sea water quality assurance at the design stage of the coastal hydraulic constructions like ports, recreational and bank protection structures is presented. The half-closed coastal water areas are the object of the intensive pollution and in the same time have the limited water exchange with the main basin. The research objective was the development of forecasting method of the water quality in the harborage and the guidelines on the required water quality supply. The study was carried out for the conditions of the yacht port “Grand-Marina Sochi”, which is being designed for placing in the area of seaport Sochi. The oil spill and the failures at the wastewater treatment facilities are considered like the sources of the pollution. The numerical modeling of flow fields has been applied to forecast the spreading of the pollution under the determined hydro-meteorological scenarios. On the basis of the study there were made recommendations on the program of the environmental monitoring of the sea water quality in the harborage.