Abstract: This study aimed to clarify how open-sea holes in aquaculture tank affect the motion responses and the internal wave field and how these effects vary with the liquid-fill ratio. A physical model with a geometric scale of 1:60 was constructed to enable direct comparison between a closed tank configuration and a perforated configuration in which the open-sea holes were oriented parallel to the incident-wave direction. Three liquid-fill ratios were examined, namely 50%, 60%, and 65%. Response Amplitude Operators (RAOs) of heave and pitch were measured together with internal free-surface elevations acquired from a spatial array of wave gauges. The time–frequency content of the internal wave field was evaluated using a continuous wavelet transform so that transient energy concentrations associated with sloshing could be resolved. Repeated runs were performed at each condition to verify measurement consistency, and the dataset covers regular waves spanning short- to long-period seas that are representative of operational environments for aquaculture vessels. The perforated aquaculture tank exhibited a pronounced double-peak behavior in the heave RAO. Relative to the closed tank, the presence of open-sea holes produced a damping effect that reduced the heave amplitude by 34.2%, 28.6%, and 33.5% at the three liquid-fill ratios. The locations of the two heave peaks were found to be aligned with the neighborhoods of the first internal sloshing mode of the liquid and the global heave-related resonance of the system, indicating coupling between internal sloshing of the liquid tank and the heave motion excited by external waves. This coupling provides a consistent physical explanation for the coexistence of a short-period peak associated with internal dynamics and a longer-period peak associated with the overall rigid-body heave, and it clarifies why the open-sea holes can reduce overall heave yet preserve a two-peak signature in the transfer function. In terms of pitch, the response was controlled by a different but related mechanism. A time-varying pressure difference across the open-sea holes generated additional side-wall loads and an additional moment, which acted as an external excitation that interacted with the first sloshing mode within the tank. This interaction amplified the pitch response and produced a clear double-peak signature in the pitch RAO. The effect was most evident under higher liquid-fill ratios condition. These findings highlight that the hydrodynamic influence of open-sea holes is not uniformly beneficial across all degrees of freedom. While open-sea holes reliably mitigate heave, they can enhance pitch through the combined action of unsteady pressure differentials and internal sloshing, a trade-off that should be accounted for in design. The open-sea holes also increased the non-uniformity of the internal wave field. Under short-period incident waves with higher liquid-fill ratios, the down-wave side of the tank consistently exhibited larger free-surface amplitudes than the up-wave side, revealing a persistent spatial bias in the distribution of wave activity. Wavelet analysis corroborated these observations by showing that the presence of open-sea holes raised the internal wave-energy content, particularly in higher-frequency bands associated with sloshing, and that the degree of enhancement diminished progressively as the incident period increased. Overall, these results indicate that open-sea holes promote energy exchange between the exterior wave field and the interior liquid, which reorganize the internal flow and lead to spatially varying free-surface responses. In conclusion, open-sea holes in a perforated aquaculture tank can effectively reduce global heave while increasing the spatial non-uniformity and the energy level of the internal wave field, with the strongest effects observed under short-period waves and higher liquid-fill ratios. In terms of engineering practice, the hydrodynamic benefit of open-sea holes should be balanced with targeted anti-sloshing provisions inside the tank. The results provide quantitative evidence and actionable guidance for the design and optimization of a perforated aquaculture vessel.