The paper studies the problem of reducing the errors of heading and pitch/roll angles for a strapdown inertial navigation system (SINS) based on fiber-optic gyroscopes (FOG) during a vessel maneuvering. The solution of the problem is analyzed mainly for autonomous mode of the system operation using the water speed log data. A specific feature of the studied solution is that the gyro drifts and accelerometer biases are estimated only during the vessel maneuvering. In this case, an attribute is formed for the vessel’s maneuver start. The results of simulation, test-bench and field tests of the SINS on FOGs of navigation accuracy grade during the vessel maneuvering are presented, with the data of the system’s measurement unit, GNSS-receiver and log having been processed in MATLAB (Simulink) software, taking into account the simulation of ocean currents and vessel drift.