This paper addresses the problem of actuator fault detection for a Remotely Operated Vehicle widely used in navy and marine industries. Firstly, the dynamical model for the Remotely Operated Vehicle is presented. Then this model is used to develop a diagnostic system based on a nonlinear (Thau) observer for residual generation, and on a sequential change detection algorithm for residual evaluation. The diagnostic system is tested through simulation trials in which a fault is injected into one of the actuators. The analysis of the results proves that the developed detection system can correctly detect a fault on one of the actuators by using only measurements of sway, surge and yaw (and their speeds), thus representing an effective solution ...