A new detection system named "KATERINA" is developed and applied for measuring marine radioactivity using NaI(Tl) as a crystal. This apparatus is designed for qualitative and quantitative radionuclide detection in the marine environment with maximum depth of deployment 400 m. A detailed study for the enclosure of the system and for the electronics development is performed. The system offers volumetric activities in Bq/m(3) using the marine efficiency calibration, which is measured by dilution of three reference sources ((99m)Tc, (137)Cs, and (40)K) in a special tank. The calibration procedures for energy, energy resolution and efficiency were analyzed with particular attention to the factors that affect the calibration parameters. The underwater detector is tested in the field for acquiring radon daughters ((214)Pb and (214)Bi) in a region where submarine groundwater discharge exists in the coastal zone.