To achieve precise excavation boundary control and promote planar cracks propagation in underwater blasting operations, this study systematically investigates fracture initiation mechanisms through the deployment of multi-point bidirectional shaped charges submerged in aqueous environments. Based on the unique energy transmission dynamics of underwater explosions, compact-volume shaped charges were engineered with forward-positioned air cavities adjacent to the liners, optimizing jet formation efficiency and directional fracture control. The actuation sequence and planar fracturing mechanism of the underwater shaped charge were comprehensively analyzed through integrated 2D and 3D numerical simulations and concrete splitting experiments. The results demonstrate that the designed actuation sequence proceeds through distinct phases: blast shockwave propagation, head shockwave formation, shaped jet generation, hydraulic wedge action, and bubble pulsation effects. The synergistic interaction of these mechanisms establishes an optimized chronological sequence that, when coupled with pre-cut borehole grooves, induces a tensile stress concentration zone along the predetermined fracture plane, facilitating preferential crack initiation and controlled propagation. Field validation tests successfully generated planar fractures in concrete targets using a remarkably low linear charge density (39 g/m), with fracture patterns exhibiting exceptional alignment with the designed splitting plane and strong correlation with numerical simulation. These findings confirm both the operational feasibility of the charge configuration and the predictive accuracy of the computational model. The developed methodology offers significant practical value for boundary-controlled blasting applications in underwater environments and in terrestrial settings requiring stringent blast-impact mitigation.