Water is central to sustainable development; making transboundary cooperation among riparians a core aspect of Agenda 2030 and the Sustainable Development Goals (SDGs). SDG 6 on water and sanitation includes an indicator, SDG Indicator 6.5.2, to assess cooperation over transboundary waters by measuring the “Proportion of transboundary basin area with an operational arrangement for water cooperation.” In this research, we analyze the methodology for assessing SDG Indicator 6.5.2, examine how operational cooperative arrangements are defined, and identify the strengths and limitations of the method for measuring transboundary water cooperation. This is done by simulating the application of the methodology and calculating the Indicator in three national case studies (Bangladesh, Honduras, and Uganda) and in a global overview.