Socio-scientific issues (SSI) are informed by science concepts but require consideration of societal aspects in order to be effectively understood and resolved. As a result, functional scientific literacy necessitates fluency with science as well as other domains of knowledge when engaged in reasoning about science and societal dimensions of SSI (i.e., socio-scientific reasoning (SSR)). However, a holistic examination of those domains of knowledge that inform a particular SSI has not been undertaken. In this investigation, thematic analysis is employed to explore domains of knowledge undergraduates (N = 91) used when reasoning about a regionally relevant SSI after completing a semester-long course about contemporary water-related issues. We found that participants used a number of knowledge domains, including science and ethics, as well as domains from the social sciences, though the number and type of knowledge domains differed within and across SSR dimensions. These findings inform SSI research and instruction in the context of SSI, as they begin to make concrete the diversity of knowledge domains with which individuals need familiarity and which must be synthesized to effectively understand and respond to SSI and thus exhibit functional scientific literacy.