Underwater blasting technology has been widely used in inland waterway improvement projects. However, due to the particularity and complexity of underwater blasting, it is difficult to predict the transmission patterns of underwater blasting shock waves. Therefore, based on the Guoyuan Port Phase II project in Chongqing, the transmission patterns and influencing factors of underwater drilling blast shock wave pressure were investigated by field monitoring and numerical simulation. In this study, a total of 45 groups of shock wave pressures were measured, and the underwater shock wave pressure transmission formula obtained through data fitting was P = 27.39 × (Q1/3/R)1.25. Furthermore, the shock wave pressure transmission process in water was numerically simulated, and the simulation results were verified using field monitoring data. The results showed that the simulation and measured results were consistent. Finally, the influence of water depth, flow rate, and flow direction on the transmission pattern of shock wave pressure was analyzed, based on a numerical simulation method. The results showed that the more blastholes there are, the smaller the peak pressure of the shock wave. The lower the depth of blasting, the faster the decay of shock wave pressure. The flow rate has less effect on the shock wave pressure. At flow rates of 1, 2, 3, and 4 m/s in the range of 0 to 50 m, the shock wave pressure in the upstream flow decreased by 5.7%, 7.4%, 9.1%, and 10.2%, respectively, compared with that in the downstream flow. This study provides a theoretical basis for safety control of underwater drilling blasting engineering in inland waterways.