Water, due to the high hydrogen content and availability, is effective and common neutron shielding. However, due to the low atomic number of hydrogen and oxygen, water is not an acceptable shield against gamma rays. On the other hand, in some cases, this disadvantage (low density) can be compensated by the high thickness of the water shield. In the case of neutrons, water perfectly moderates neutrons, but with the absorption of neutrons by hydrogen nuclei, secondary gamma rays with high energy are produced. These gamma rays highly penetrate matter, and therefore they can increase requirements on the thickness of the water shield. Adding a boric acid can help with this problem (neutron absorption on boron nuclei without strong gamma emission) but results in other problems with corrosion of construction materials.
See also: Shielding of Neutrons