In this study, an accurate faster gamma spectrometry method for measuring the low level activity concentrations of 137Cs using in situ pre-concentration technique on copper ferrocyanide cartridge was standardized. Due to unavailability of reference standard in the copper ferrocyanide matrix, efficiency calibration curves were plotted using RGU and RGTh reference standards. To harmonize the difference in density of standard and sample the required density correction factors for photo peak efficiency were generated. The in situ pre-concentration technique followed by gamma-ray spectrometry was applied for activity determination in surface seawater from eight locations in the coastal marine environment of Arabian Sea. The mean activity concentration of 137Cs ranged between 0.71 and 0.91 Bq/m3. Higher activity concentrations were observed at location with latitude, longitude of 21.6 °N, 69.57 °E as compared to concentration observed at location with latitude, longitude 16.98 °N, 73.25 °E. The observed concentrations were found to be in range of data reported in Asia–Pacific Marine radioactive database (ASPARMARD). The results will fill up the gaps in the existing database. The generated data will be useful for monitoring fresh input of anthropogenic radionuclide into coastal marine environment for post Fukushima environmental assessment.