A current trend in marine robotics consists of performance evaluation of Unmanned Marine Vehicles (UMVs) guidance systems. This paper contributes to this by defining and testing performance indices and metrics to quantitatively measure and compare path-following performance. It focuses on the definition of a new criterion for evaluating the capability of an Unmanned Surface Vehicle (USV) to follow a desired generic curvilinear path. The main advantages of the method are represented by the possibility to compute the proposed index online and in a very general situation. Theoretical bases of the presented approach and preliminary results on simulated data that highlight the versatility of this criterion are presented.