The design of interactive applications for online communication is an ongoing area of research within technical communication. This study reports on the development of an interactive sea-level rise (SLR) viewer, a data visualization tool that communicates about the potential effects of SLR along coastlines. It describes the formative evaluation of a location-specific SLR viewer created via integral stakeholder engagement. Participants performed a series of tasks, answered questions about the tool's usability and communicative effectiveness, and made suggestions for ways to improve its application to desired tasks. The authors discuss the implications of this study for visual risk communication and make recommendations for others developing similar interactive data visualization tools with audience input.