A cornerstone ability of an autonomous unmanned surface vessel (USV) is to avoid collisions with stationary obstacles and other moving vehicles while following a predefined path. USVs are typically underactuated, and this paper extends recent results in set-based guidance theory to an underactuated surface vessel, resulting in a switched guidance system with a path following mode and a collision avoidance mode. This system can be used with any combination of path following and collision avoidance guidance laws. Furthermore, a specific guidance law for collision avoidance is suggested that ensures tracking of a safe radius about a moving obstacle. The guidance law is specifically designed to assure collision avoidance while abiding by the International Regulations for Preventing Collisions at Sea (COLREGs). It is proven that the USV successfully circumvents the obstacles in a COLREGs compliant manner and that path following is achieved in path following mode. Simulations results confirm the effectiveness of the proposed approach.