Optimal path planning is an important part of mission management hierarchy in a modern unmanned surface vehicle (USV) guidance, navigation and control frame work. USVs operate in a complex dynamic marine environment comprising of moving obstacles and sea surface currents. These characterising variables of configuration space change spatially as well as temporally. The current work investigates a well-known search technique, the Dijkstra algorithm, to resolve the problem of motion planning for a USV moving in a maritime environment. The current study extends the implementation of Dijkstra algorithm in a space cluttered with static and moving obstacles. In addition, downstream and upstream effects of sea surface currents of different intensities on optimal path planning are studied. The performance is verified in simulations with total path length and elapsed computational time considered as parameters to determine the effectiveness of the adopted approach. The results showed that the approach is effective for global path planning of USVs.