In-situ underwater gamma-ray spectrometer KATERINA was used for continuous measurements of radon progenies (214Pb, 214Bi), thoron progeny (208Tl) and 40K in submarine groundwater discharge (SGD) sites at Kalogria Bay, SW Peloponnesus (Greece). The spectrometer was deployed attached on measuring platform along with two conductivity - temperature data loggers while underwater battery packs supplied the system for acquisition periods up to 25 days. The radionuclide time series together with salinity data were obtained for spring (wet) and summer (dry) seasons. The 40K activity concentrations correlated well with salinity of the emanating groundwater. Although the 214Bi and 208Tl activities showed usually similar trends anticorrelating with salinity, in some cases 208Tl did not follow the 214Bi record due to changes in the dynamics of the groundwater aquifer. As the half-life of 220Rn is very short (55.6 s), its concentration in SGD may depend on the distance from its origin to the monitoring point. The observed temporal variations of 214Bi and 208Tl confirmed advantages of continuous in-situ monitoring of SGD in coastal areas.