This paper studies the transient performance improvement problem for path following control of underactuated surface vessels (USVs) in the presence of oceanic disturbances. The traditional practice that chooses the tangent direction of the desired path as the desired heading may deteriorate the tracking performance in the curve-path following. That is because the sideslip angle is not zero in turnings, which unavoidably makes the lateral offset hard to converge to zero. Also, the disturbances in wave filed greatly affect the transient control of the path following errors. To this end, this paper makes two contributions: 1) An amendment on the choice of the desired heading is presented to consider the sideslip angle in turnings and then achieve a more accurate path-following maneuver; 2) A novel robust composite nonlinear feedback (CNF) technique is proposed based on a multiple-disturbances observer to improve the transient performance for path following control in seaway environment considering the input saturation. Comparative simulations verify the reasonability of the amendment on the desired heading direction and the effectiveness of the CNF approach in improving the transient performance for the path following control of USVs.