Heavy coastal precipitation is traditionally attributed to atmospheric dynamics, but the influence of co-occurring oceanic thermal extremes remains largely unexplored. Using satellite observations and reanalysis data from 1998 to 2023 over the South China Sea (SCS), we investigate the synergistic impacts of marine heatwaves (MHWs) and atmospheric boundary layer jets (BLJs). We reveal a compound effect: when MHWs and BLJs co-occur, regional coastal precipitation increases by 47.8% relative to climatology, more than doubling the 22.3% increase during pure BLJ events. Moist static energy diagnostics demonstrate that MHWs act as a thermodynamic catalyst, enhancing surface evaporation by 20%-30% and boosting low-level atmospheric latent energy. The concurrent BLJ functions as a dynamic conveyor, rapidly transporting this excess moisture landward to fuel heavy coastal rainfall. These results imply that assessing coastal precipitation from atmospheric dynamics alone may largely underestimate the rainfall response when marine heatwaves are present, a coupling set to strengthen as the ocean warms.