Abstract: The Inertial Navigation System (INS) is usually employed to determine the position of an underwater vehicles, like Remotely Operated Vehicles (ROV) and, more recently, Autonomous Underwater Vehicle (AUV). The accuracy of the position provided by the INS, which uses accelerometers and gyroscopes, deteriorates with time. An external aiding sources such as the Global Positioning System (GPS) can be employed to reduce the error growth in the INS. The GPS aided INS system provides enhance positioning accuracy of the underwater vehicles compared to that of a stand-alone INS technique. In the paper integration algorithm of inertial sensors (accelerometers and gyroscopes) and GPS system data for underwater navigation is presented. For data integration algorithm External Kalman Filter (EKF) is proposed.