Numerical predictions of the acoustic field inside the embedding polymer body of a submerged circular-disc shaped fiber-optic hydrophone are presented. The field is excited by a plane wave insonifying the disc, modelled as a fluid body with material parameters for polyurethane. The field is computed numerically by a frequency-domain, boundary integral equation technique for a set of frequencies in the range 1 kHz to 100 kHz. At frequencies up to approximately 10 kHz, the disc is found to be acoustically transparent. At higher frequencies, the interior field deviates increasingly from the incident one. This deviation is beneficial, in the sense that the sensitivity of an embedded hydrophone array is increased without degradation of the directivity.