Rayleigh scattering induces serious coherent noise in remotely interrogated fiber-optic interferometric hydrophones especially when high coherence laser sources are used. The impacts of Rayleigh backscatter (RB) and double Rayleigh scattering (DRS) on hydrophone systems are analyzed and optical isolators are used to eliminate noise due to RB. Then the phase noise due to DRS and its characteristics are investigated, and it is inferred that phase generation carried (PGC) technique can suppress these coherent noises besides its general function of overcoming bias induced signal fading. In an experimental setup with 25 km input and output fibers, a maximal 20 dB decrease of phase noise is achieved by the use of isolators. A further decrease of maximal 10 dB is achieved by PGC technique. Thus when RB and DRS are suppressed simultaneously, Rayleigh scattering induced coherent noise in a 50 km system is almost eliminated.