The goal of underwater docking is to safely insert an autonomous underwater vehicle (AUV) into the docking sleeve of a docking station (DS). However, AUVs frequently experience disturbances in their operating environment under motional constraints owing to their shapes, which can significantly impede successful docking missions. Therefore, it is essential to develop an assessment method and corresponding index representing feasibility. In this study, we suggest a new assessment method and a probability-based assessment index to assess the underwater docking process, considering aforementioned motional constraints. The assessment is made for both the position and heading angle of the AUV, with the results presented in probabilistic figures. These figures are used to estimate the assessment index, which represents the probability of successful docking. The final decision on whether to dock or not can be made based on this index. When the index exceeds a predefined threshold, it indicates that the current docking process is reliable, and the docking will be successful. The suggested assessment method and the index were validated through tests conducted in various underwater environments. The results show that the probability-based index estimated through the proposed method can be grounds for successful docking.