Data management and communications within the marine environment present great challenges due to the variety and complexity of the observations that are involved. At present there is no coherent data management strategy that can effectively integrate these data streams across disciplines and spatial and temporal scales. The resulting lack of integration of data denies US society important benefits, such as improved climate forecasts and more effective protection of coastal marine ecosystems. Therefore, Congress has directed the US marine science communities to come together to plan, design, and implement a sustained Integrated Ocean Observing System (IOOS). Central to the vision of the IOOS is a Data Management and Communications (DMAC) Subsystem that joins Federal, regional, state, municipal, academic and commercial partners in a seamless data sharing framework. The design of the DMAC Subsystem is made particularly challenging by three competing factors. 1) The data types to be integrated are heterogeneous and have complex structure; 2) The holdings are physically distributed and widely ranging in size and complexity; and 3) IOOS is a loose federation of many organizations, large and small, without a corporate management hierarchy. Designing the DMAC Subsystem goes beyond solving problems of software engineering; the most demanding aspects of the solution lie in community behavior. An overview of the plan for the DMAC Subsystem is described.