Abstract: There is provided a fiber-optic hydrophone having a compliant sensing mandrel coaxial with and adjacent to a rigid reference mandrel. A first optical fiber is wound around the compliant sensing mandrel and a second optical fiber is wound around the reference mandrel. The first and second optical fibers comprise different arms of an interferometer. Flexible sealing members, such as O-rings, seal the compliant sensing mandrel to the hydrophone. One O-ring is disposed near each end of the sensing mandrel. A cylindrical support member is disposed inside the sensing mandrel. At least a portion of the support member is spaced from the sensing mandrel so as to provide a sealed cavity between the sensing mandrel and the support member. The sealed cavity is filled with air or similar compliant substance.