An attempt has been made to detect inhomogeneities in the water vapor content in the vicinity of the coastline of the Gulf of Finland using microwave observations by the RPG-HATPRO radiometer which are performed in angular scanning mode in the direction of the water body. The radiometer is located at a distance of 2.5 km from the coastline. Numerical simulation of the experiment has shown that the contribution of the horizontal inhomogeneity of the water vapor content to the measured brightness temperature at large zenith angles can be higher than the value of the random error of the instrument by the factor of 10-100. Experimental data were analysed for the period from May to October for two years of observations: 2013 and 2014. The results have shown the presence of a seasonal variation of the difference between the water vapor content over land and sea: in May, there is more water vapor over land than over the sea; in June, July, August and October, the situation is reversed. There is an uncertainty for September.