<p>The dynamic stiffness characteristics of composite-to-steel joints play an important role in the underwater vibro-acoustic performance of submerged composite-steel structures, yet their quantitative influence under fluid loading remains insufficiently understood. This paper investigates the differences in underwater vibro-acoustic responses between two representative connection types—bolted and embedded joints—using experimentally calibrated numerical models to examine their frequency-dependent characteristics. A research strategy combining experimental calibration and numerical prediction is adopted, in which the stiffness characteristics of the joints are first identified through vibration transmission loss experiments in air. Subsequently, the underwater acoustic radiation of full-scale grillages is predicted using an acoustic–structure coupling algorithm. The results indicate that the embedded joint exhibits higher vibration transmission loss than the bolted joint, implying a stronger effective dynamic constraint. In the underwater medium-frequency range, this higher stiffness shifts the structural resonance frequencies to higher values, whereas the acoustic responses of the two configurations remain similar in the low-frequency range due to the dominance of global structural modes. In the higher-frequency range, although the bolted joint exhibits sharper peaks in mean square velocity at certain frequencies, the difference in radiated sound power between the two connections becomes relatively small, suggesting reduced sensitivity of acoustic radiation to further increases in boundary stiffness under the present conditions. Overall, the results provide a quantitative reference for joint design and vibration–noise control of submerged composite-steel structures in marine engineering applications.</p>