Shipboard Command and Control (C2) presents unique challenges for decision support. Tactical decisions require that the ship's Command Team be able to gain timely access to and comprehend the significance of large amounts of information that may impact the mission. While operations depend heavily on doctrine and standard operating procedures, many tactical details must be established in real time, particularly as unanticipated events or anomalous situations arise. This imposes significant cognitive demands on operators for active situation assessment and decision making, where benefits for performance improvements may be expected from incorporating advanced decision support systems and integrative work aids and displays. Cognitive Work Analysis (CWA) is a layered, systems-based analysis framework that specifically addresses system design to support operators in unanticipated situations. Its first layer is a work domain analysis (WDA), which develops hierarchical models representing the intentional, functional, and physical properties of the work domain, at different levels of abstraction, as well as the relations between these levels. In this paper, we discuss WDA's basic concepts and their application in a year-long study to tactical C2 for the Canadian Navy's HALIFAX Class frigate. We review the models that resulted and briefly describe some ongoing applications of the CWA approach in this work environment.