In recent years, there has been a growing interest in self-guided vessels, for a wide range of application domains such as scientific research, ocean resource exploration, transport and other. In this paper we explore one of the most critical components of autonomous vessels, intelligent transportation and motion planning, and propose an automatic collision avoidance methodology, applicable to both unmanned surface or underwater vessels operating on the sea surface, using Dynamic Visibility Graphs while respecting International Regulations for Preventing Collisions at Sea (known as COLREGS). The efficiency of the developed method is investigated and discussed through characteristic simulated experiments.