Set up and application of a stationary monitoring network for measuring specific gamma- activities in the Aegean Sea are described. Three NaI scintillator based spectrometers have been used to detect the gamma rays. The gross counting rate of each system was found to be nearly constant, when there was no rainfall. The volumetric activity of the natural gamma-ray emitter 40K in open sea varied from 12,200 to 13,000 Bq/m3. The counting rate for 1461 keV 40K radiation was measured by intercalibration with an appropriate salinity sensor mounted close to the NaI-detector system. A simple relation between the counting rate and the salt concentration has been observed. The amount of the artificial radioactivity from 137Cs was increased up to seven times higher after strong rainfall, compared to the radiation level as given in literature (3.5-5.5 Bq/m3), while the 214Bi counting rate was increased up to ten times compared to the data without rainfall.