For meteorology the precipitable water vapor becomes a more and more important product in the data analysis of the Global Positioning System (GPS). It is derived from the wet part of the tropospheric refraction obtained as zenith path delay in the data analysis. However, the estimates of the zenith path delay and of the station height are strongly correlated. The ocean loading effect has a periodic influence on station heights with major frequencies in the diurnal and semidiurnal band and with amplitudes up to several centimeters. If one ignores this effect it affects the estimates of the zenith path delay and thus the precipitable water vapor. This will be demonstrated by some examples.