This paper provides an overview of the development of an interactive simulation system and Graphical User Interface (GUI) for an underwater robotic vehicle. Computer simulations and graphical interfaces are effective ways for studying the behavior of a system well before it is developed and deployed. It allows lot of flexibility in the design and greater scope for improvement without major financial commitments. The underwater vehicle and its sub-system properties, control inputs (parameter adjustments, maneuvers, etc), and the underwater environment are graphically represented on an user interface that can be easily accessed by the user for modifications. The interface provides convenient means to evaluate vehicle performance by means of animations, dashpot and time history plots of engineering variables of interest. The dynamic model and control algorithms, developed using standard tools are used at the back-end of the program to model and simulate various control behaviors of the vehicle. The environment conditions, parameter variations, disturbance forces etc. can be simulated using the back-end algorithm. The simulation results can be visualized using the GUI and various control strategies, navigational algorithms and path planning can be easily tested using the interface program. Few examples showing how the simulations can be used to identify design changes that can improve underwater vehicle dynamic and control performance are also provided. Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE).