Please be more specific. How does one count the water sample, and what is the radionuclide one is counting? One can use the gamma spectrometry to determine the radionuclide(s) and from that perhaps make some determination of the concentration of the radionuclide in the water. One can also assume uniform dispersion in the water. So one can determine counts per sec and energy/disintegration. There will be some self-shielding involved, so one integrates over the energies of the spectrum. One rad is defined as an absorbed dose of 0.01 joules of energy per kilogram of tissue. One gray is defined as 1 joule of deposited energy per kilogram of tissue, 1 gray = 100 rads. For gamma and X-ray, the biological effectiveness is taken as unity, so 1 rem = 1 rad, or 1 sievert = 1 gray (1 sievert = 100 rems). Refer to Hyperphysics - radiation risk for units - http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html and http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/radexp.html