Ships equipped with a dock are essential assets for the deployment and recovery of underwater equipment. However, when these ships operate in complex sea conditions, significant fluid sloshing can easily occur inside the dock, jeopardizing operational safety. Therefore, accurately predicting the fluid sloshing frequency inside the dock is of great importance. This study compares the natural frequencies predicted by an analytical model with those computed using the hydrodynamics software WAMIT. The results demonstrate that the relative errors between the analytical predictions and WAMIT simulations are within 5%, and these errors decrease as the resonant mode order increases, validating the reliability of the analytical model. The wave height distribution inside the dock and the longitudinal sloshing modes corresponding to each natural frequency are also analyzed. By comparing the dock's free-surface response under both freely floating and fixed hull conditions, the influence of hull motion on the natural frequencies of the internal fluid in a dock is investigated. It is found that hull motion in following seas reduces the longitudinal natural frequency of the dock, whereas in beam seas, it increases the natural frequency. Additionally, the study confirms that internal dock sloshing significantly affects the motion response of the ship.