The maximum potential for underwater exploration rests within the use of multiple Autonomous Underwater Vehicles (AUVs) and tasks involving human diver-AUV coordination. Such missions are gaining increasing popularity with the advent of better control mechanisms and availability of acoustic modems. However, high costs and the lack of useful tools to simulate multi-AUV tasks have hindered the full potential of the field. Though simulators could aid the development of AUV communication systems to help such missions, the few existing simulators focus upon simulating a single vehicle and, as such, do not provide tools for simulating underwater communication systems. In this paper we present an overview on modeling of the underwater acoustic channel, taking into account the high degree of local variability of ocean conditions, multi-path echoes and ambient noise, within the framework of an underwater acoustic communications server for the Unified System for Automation and Robotics Simulator (USARSim) robotics simulator, a simulation tool is capable of modeling multi-robot missions with high accuracy.