Abstract: To address the practical engineering challenge wherein low-temperature deep-sea environments increase the viscosity of heavy oil in shipwrecks and impede recovery operations, this study establishes a fluid-thermal coupled numerical model based on the enthalpy-porosity theory. This model accurately characterizes the latent heat of solid-liquid phase change and the rheological properties of non-Newtonian fluids. Comparative analysis reveals that the latent heat of phase change is a critical factor governing the evolution of the flow field. If latent heat is neglected, the model erroneously predicts a stratified distribution of heavy oil dominated by horizontal diffusion, with the high-temperature region (>295 K) confined entirely to the bottom area near the heating rod. Conversely, when latent heat is incorporated, a low-viscosity (<50 kg/(m·s)) thermal plume overcomes high-viscosity constraints via buoyancy and ascends vertically to the upper region, confirming that natural convection is the dominant mechanism driving heavy oil flow. Furthermore, an analysis of the "heating dead zone" at the tank bottom indicates that the extremely low thermal conductivity of solid heavy oil impedes heat transfer, restricting the overall melting efficiency. Numerical simulation results demonstrate that increasing the steady-state heat source temperature from 30 °C to 70 °C significantly accelerates the ascent of the central thermal plume but does not noticeably reduce the area of the bottom heating dead zone. This finding indicates that the extent of the dead zone is primarily dictated by the physical properties of the fluid and the flow field structure, rather than solely by the heating power. Ultimately, this research elucidates the multi-field coupling mechanisms underlying the heating process of heavy oil in shipwrecks, providing a robust theoretical foundation and engineering reference for determining heating power requirements and optimizing multi-point heat source layouts in deep-sea salvage operations.