The traditional method for measuring sound velocity in seawater is developed on the basis of the piezoelectric effect. This traditional method is prone to errors because of its uncertain starting points when measuring the propagation time and distance. To address these problems, this study proposes an approach for measuring underwater sound velocity based on the acousto-optic effect between optical frequency comb and ultrasonic pulse. We introduce the acousto-optic effect with a clear interaction point between sound and laser. A dual Michelson interferometer system is built to measure the optical pulse markers generated when the ultrasonic pulse passes through two measuring arms. We measure the ultrasonic propagation time and distance using the cross-correlation technique. The traceability of the method is clear, however, the accuracy of the method has room for improvement. The experimental results show that the experimental system can achieve high precision measurement of underwater speed, and the measurement uncertainty is 0.023 m/s. It can be used as a new calibration device for measuring underwater sound velocity.