Underwater communication networks (UCN) are enablers of current and future military and commercial applications involving undersea exploration, monitoring, and surveillance. Unmanned Underwater Vehicles (UUVs) hold promise for stealthy low-cost surveillance, particularly in areas where manned undersea platforms cannot be easily deployed, operated, and maintained. Aiming to extend the operational lifetime of UUVs, underwater hubs are being developed for charging UUVs and exfiltrating data from UUVs. As the number and heterogeneity of undersea platforms and infrastructure expected to operate in concert continuous to increase, command, control and coordination of undersea platforms, and timely and secure transfer of data become increasingly important. UCNs face an inherently disconnected, interrupted and low-bandwidth operational environment and often require specialized protocols for supporting reliable transfer of data. Furthermore, different communication technologies being considered for undersea applications, such as acoustic, optical, and RF communications, have different, yet complementary, characteristics in terms of propagation delay, communication range, and bandwidth. Characterization of the performance of a UCN is critical to avoid unexpected operational disruptions. Yet, in-water experimentation is restrictive and costly, and often cannot be performed in an environment similar to where the UCN deployment is planned. Network modeling and simulation tools offer a low cost alternative for characterizing the performance of a UCN. Unfortunately, most commercially available network simulators designed for modeling wired and RF-based wireless networks are not well suited for modeling UCNs. This paper describes UCN-X, a scalable UCNs simulator that can be used to investigate real-time underwater command and control, data transfer and exfiltration. UCN-X leverages parallel discrete-event model execution and system-in-the-loop interfaces from EXata, to provide a scalable, live, virtual, constructive (LVC) UCN simulator that can interface with live modems and mission operation and management software applications. UCN-X captures undersea signal propagation effects obtained based on the characteristics of the undersea propagation environment and a diverse set of protocol models at all layers of the communication stack. It additionally includes store-and-forward protocol, data muling, and air/water gateway node models. To support its use within a system-of-system context, UCN-X can be federated with other simulators including force-force training environments to provide realistic undersea communication modeling. UCN-X can also be interfaced with live and unmodified UUV command-and-control (C2) software. By providing a realistic representation of the dynamic network performance under realistic operating conditions, such a federated model provides a rich and extensible training capability for both military and commercial applications.