The development and application of a new detection system is described for autonomously working radioactivity measurements, usable in seawater and river environment. The system is based on a NaI scintillator with specifications for use in the marine environment and for real time acquisition. It is simple, stable for long-term monitoring, and of low consumption. The sensor was energy and efficiency calibrated in the laboratory. Many tests were made for the linearity and the stability of the electronics. The energy resolution calibration of the sensor was performed prior to its location into the tank, using various reference point sources. The system was deployed in open sea in order to measure background and low volumetric activity of 137Cs (19 Bq/m3). The field measurements in the Aegean Sea offer very promising results concerning the use of the whole system in the marine environment to be used as continuous monitoring and alarm system.