An axial flatted mandrel type of fiber optic hydrophone, named as Mark VI, has been constructed and tested. The sensor is based on a Michelson type interferometer and is built with polarization-maintaining fiber optic components to suppress signal fading due to changes in polarization. The sensitivity is tested in a range from 200 Hz to 1500 Hz. Up to 1000 Hz the sensitivity is found to be 275 mrad/Pa or higher. With 0.8 m of sensing fiber the normalized sensitivity is found to be -277 dB re 1/(mu) Pa, which is one of the highest ever reported. Up to 1000 Hz the noise level of the Mark VI is about 10 dB below Seastate Zero.