Water present in nuclear legacy materials, such as spent nuclear fuel and fuel element debris, can impact both the chemical and structural stability of these materials. Subsequently, the suitability of these materials for disposal in geological repositories is degraded. Water ingress increases the potential for radioactive material to be dispersed and for neutronic properties to be augmented due to changes in neutron moderation by the constituent hydrogen. This risk necessitates the detection and quantification of water in what can be complex nuclear contexts. Neutron capture γ-ray analysis is of particular interest in this regard: this review provides a summary of the methods and equipment used to apply this to the problem of water assay in nuclear materials. Whilst relatively few directly relevant reports have been published concerning the measurement of water in spent nuclear fuel, the detection of hydrogen and the measurement of its characteristic 2.223 MeV γ-ray following the 1H(n,γ)2H reaction has been studied more extensively. This approach for detecting water in composite materials and its impact on PGAA of other materials is considered. The review concludes with the current applications of the capture γ-ray analysis technique and the potential of its use for water ingress assay nuclear materials at Fukushima Daiichi and Chernobyl.