Accurate prediction of towing resistance is essential for selecting tug capacity and controlling energy demand during the wet-towing of floating offshore wind turbines (FOWTs). This study established a high-precision CFD numerical wave tank to simulate the towing hydrodynamic performance of the IEA-15 MW semi-submersible FOWT under calm water and regular wave conditions, covering two typical towing layouts (single-column-forward and double-column-forward) and three representative towing speeds. Based on CFD free-decay results, viscous damping corrections in heave, roll, and pitch were applied to the AQWA, while the drag coefficients were introduced through Morison elements. On this basis, an integrated viscous-potential flow coupled numerical model for the tugboat-FOWT system was then established. Frequency-domain analysis showed that wave excitation loads increase with towing speed under following-wave conditions, while the reverse trend occurs under head-wave conditions. Time-domain validation demonstrated that the viscosity-corrected potential flow model achieves a deviation below 2% for calm water resistance and less than 5% for mean wave resistance compared with high-fidelity CFD benchmarks. Parametric analysis under multiple regular wave conditions revealed that wave-added resistance first increases and then falls as wave period increases, consistent with the variation in surge wave excitation forces, and exhibits a nonlinear growth pattern with wave height. The double-column forward layout yields overall higher towing resistance than its single-column alternative, while main towline tension can accurately reflect the total towing resistance level. These technical outcomes provide actionable engineering guidance for low-carbon, cost-effective wet-towing scheme formulation, facilitating the sustainable, large-scale exploitation of deep-sea wind resources and advancing the long-term decarbonization of the marine energy sector.