In this study, we analyzed the effectiveness of a data visualization tool (dashboard) designed to provide insights to decision-makers about the vulnerability of water resources in the tri-national Upper Lempa River Basin (CARL) in the face of future climate and land use uncertainties. The effectiveness of the dashboard was assessed using three methods: (1) a user survey for evaluating dashboard clarity, completeness, and ease of use using seven parameters proposed by Stephen Few. The result of the survey overwhelmingly indicated a positive experience when interacting with the dashboard; (2) pre- and post-use tests were used to assess knowledge acquisition. The users’ correct answers increased by an average of 35%, and incorrect answers decreased by an average of 25% for questions assessing an understanding of water demands, the effects of climate change on the basin’s water security, land use trends, and the sub-basins with the highest vulnerability in the region; and, significantly, (3) user reports on insights drawn from their interaction with the dashboard. Users drew insights on the extent to which different regions will likely experience increased vulnerability regarding water resources and on strategies that could reduce this vulnerability. The key issues that need to be addressed to ensure that the dashboard fosters greater transparency and public participation in governance and is used by decision-makers to improve the management of water resources in this watershed are the following: (1) the incorporation of user feedback and the inclusion of adaptation strategies and their impacts on the dashboard; (2) the commitment and training of a local institution to host and maintain the dashboard and to make it available to the public; (3) the engagement of decision-makers from the three countries regarding the use of the dashboard to understand future uncertainties and the potential impact of adaptation strategies on performance metrics.