Sea-based rocket launches have become a critical capability in aerospace engineering due to their operational flexibility and expanded safety zones. However, the interaction between high-temperature supersonic exhaust and the ocean surface poses severe challenges to the structural integrity of launch platforms. Unlike land-based launches, the marine environment presents a deformable boundary with complex phase-change dynamics. This study employs the Volume of Fluid (VOF) method coupled with the k-ω SST turbulence model to conduct a comprehensive numerical investigation of these multiphase interactions. Distinct from classical internal combustion instabilities, this research reveals an external "shock–vortex–thermal" coupling mechanism formed among exhaust shockwaves, shear layer vortices, and seawater vaporization. Simulation results demonstrate that under low launch altitude conditions, rapid expansion of high-pressure steam significantly alters the shock structure, generating strong reverse jets that impose extreme thermal and mechanical loads on the platform foundation. Based on flow field topological evolution patterns, critical safety boundaries are identified. Below this altitude threshold, coupled effects trigger hazardous splashing and pressure feedback; above it, interactions effectively decouple. The parametric analysis further reveals how varying launch altitudes influence the evolution of key flow features including shock standoff distance, steam plume geometry, and pressure distribution on the platform surface. These findings provide theoretical guidance for optimizing initial launch altitude to mitigate adverse jet effects, thereby ensuring structural safety and operational stability of offshore launch platforms.