Offshore island and nearshore energy systems in southern Vietnam remain constrained by diesel dependence, fuel-logistics risks, and carbon emissions. This study applies a Delft3D-WAVE/SWAN framework forced by hourly ERA5 winds to quantify significant wave height (Hs), peak period (Tp), and finite-depth wave-power density at six strategic locations over 1995–2025. Model skill was evaluated against in situ acoustic wave and current observations. Hs performance was strong at Ca Mau (R = 0.90; Willmott d = 0.93) and moderate at Ben Tre (R = 0.70; d = 0.60), whereas Tp skill was weaker, especially at Ben Tre; wave-period-dependent results are therefore interpreted cautiously. A sharp east–west resource gradient was identified: Phu Quy had the highest mean power (13.87 kW/m) and P₉₅ (60.10 kW/m), followed by Con Dao and Tra Vinh. Significant positive wave-power trends occurred at Phu Quy (+0.1436 kW/m/yr; p = 0.005), Tra Vinh (+0.0795 kW/m/yr; p < 0.001), and Vung Tau (+0.0563 kW/m/yr; p < 0.001). The dominant directional sector contributed 56.4–73.5% of annual energy at the eastern stations and exceeded 70% at Phu Quy and Con Dao. An equal-weight, min–max-normalized Long-term Suitability Score ranked Phu Quy and Tra Vinh first and second; both positions were stable when each criterion weight was varied by ±20%. The findings provide a multi-decadal resource-screening basis for marine spatial planning, while the device yield, future-climate response, and hybrid-system performance require dedicated techno-economic and scenario analyses.