From Ohm

Example Attenuation Coefficients

Water has the attenuation coefficient $\approx 0.15cm^{-1}$.
Meaning that $D_{\frac12_{H_2O}}=\frac{ln 2}{0.15cm^{-1}} \approx 46mm$.

Strictly speaking, the attenuation coefficient is not constant but rather dependent on the energy of the gamma radiation. In the following the attenuation coefficient (in $cm^{-1}$) given for a selection of materials and gamma-energies ranging from 100 over 200 to 500 keV:

Absorber100 keV200 keV500 keV
Air0,0001950,0001590,000112
Water0,1670,1360,097
Carbon0,3350,2740,196
Aluminum0,4350,3240,227
Iron2,721,090,655
Copper3,81,3090,73