In this paper, we describe a hierarchical system for path planning and obstacle avoidance for totally autonomous Unmanned Sea Surface Vehicles (USSVs). The proposed system is comprised of three major components: a wide-area planner based on the A* graph-search algorithm, a local-area planner based on our low-resource path-planning and obstacle avoidance algorithm GODZILA, and an inner-loop nonlinear tracking control law. The performance of the proposed system is demonstrated through simulations using our high-accuracy real-time Six Degree-of-Freedom (DOF) Hardware-In-The-Loop (HITL) simulation platform whose design and implementation have been documented in our recent papers. The HITL platform is capable of simultaneously simulating multiple USSVs and passive obstacles and incorporates a nonlinear dynamic model of the USSV including detailed characterizations of hydrodynamic effects, emulation of sensors and instrumentation onboard the USSV, and the actual hardware and software components used for USSV control in the experimental testbed. The performance of the inner-loop controller has been validated through experimental tests which are described briefly in this paper and the experimental validation of the complete obstacle avoidance system is currently underway.