A detailed six degree-of-freedom (DOF) nonlinear dynamic model of an Unmanned Sea Surface Vehicle (USSV) is developed. Models of the significant hydrodynamic effects including radiation-Induced and damping forces and moments are incorporated. Disturbance models of forces and moments caused by ocean currents, waves, and wind are also included. Detailed models of the actuators including thrusters/propellers and control surfaces are included. To illustrate the importance of a six DOF modeling and the impact of disturbances, simulation results with two different controllers are presented. The detailed mathematical modeling in this paper is intended to provide the appropriate framework for a control design which explicitly addresses the six DOF USSV dynamics along with the disturbances.