A novel fiber Bragg grating (FBG) hydrophone array is fabricated by using a multi-wavelength tunable fiber ring laser, which enables simultaneous measurement of multiple underwater acoustic waves at the arrayed hydrophones. The interrogation technique of the FBG hydrophone to be arrayed is based on an intensity-modulation scheme, in which an optical wave with a narrow spectral line-width is used as an optical source, with its wavelength tuned to a slope of a reflection spectrum curve of a sensing FBG. The light reflected at the sensing FBG is modulated by an underwater acoustic wave and is converted into an electric waveform signal using a photodetector. In order to construct the FBG hydrophone array, a multiwavelength tunable fiber laser is devised for providing multiple narrow line-width optical waves and the sensing FBGs are serially arranged based on a wavelength-division multiplexing (WDM) technique. In the experiment, simultaneous detection of underwater acoustic waves at three points and measurement of sound velocity in water were successfully demonstrated.