The spectrum of acoustic pressure was measured as a function of excitation level of the transducer in distilled water, salt water, and seawater using an acousto-optic detection system. A cylindrical piezoelectric transducer was used. The sensing beam of an optical heterodyne interferometric system passes through the center of the transducer where cavitation is produced, and the acoustic pressure is detected. The acousto-optic detection does not disturb the acoustic field. The heterodyne output was demodulated and observed by the spectrum analyzer. Cavitation thresholds of seawater and distilled water were compared by observing the spectral change against the excitation level. The dip of the spectral level of harmonic components or the emergence of subharmonic and anharmonic components is characteristic of cavitation inception. The measurements were conducted in both cases: One is the case where the sensing beam passed through the cavitation site and the other is the case where the sensing beam passed near the cavitation site.