<p>Under damaged, gas-leakage, or secondary-explosion conditions, the water in liquid-containing structures, such as liquid cabins and oil tanks, may change from a liquid phase to a gas–liquid two-phase environment with dispersed microbubbles. This change can affect underwater explosion bubble evolution and bottom load transmission. To reveal the influence of an open aerated background on explosion-bubble dynamics and structural response, a transparent cylindrical water-tank experimental system was established. A 400&#xa0;V high-voltage spark discharge was used to simulate the underwater explosion process. The water depth was fixed at <i>H</i> = 175&#xa0;mm, with five relative aeration levels from non-aerated to maximum. Experiments and numerical simulations were combined to analyze bubble evolution and bottom dynamic response under different aeration levels. Results show that the maximum equivalent radius varies non-monotonically with aeration level, while the overall variation remains limited. However, it weakens the integrity of the main bubble in later cycles. The later-stage bubble evolution becomes dominated by local residual structures earlier. The maximum equivalent radius varies non-monotonically with aeration level, but the overall fluctuation is limited, from 18.65 to 21.30&#xa0;mm. The bottom load response also shows non-monotonic characteristics. Under the moderate aeration condition (0.50<i>Q</i>_max), the positive pressure peak reaches the maximum of 0.049&#xa0;MPa. Higher and maximum aeration levels reduce the load transmission efficiency to the bottom plate. These results show that background aeration is associated with coupled, non-monotonic changes in bubble pulsation, local pressure response, and bottom-plate vibration in the finite open domain. The findings can provide a reference for blast safety assessment of liquid-containing structures under leakage or aeration conditions.</p>