[en] The invention is suitable for use in gamma-activity determination of infinite volumes of water, e.g., in direct measurements of natural and artificial radioactivity of natural waters. A gamma emitting source is immersed into a sufficiently big but limited volume of water. The detector is also immersed and then the counting rate is measured. The water volume is big enough to absorb the external gamma radiation. A sealed point gamma source is used. Some advantages of the method are claimed: constancy of external gamma background; avoiding of radiation contamination of environment and personnel; higher adequacy of calibration conditions