This paper presents a series of experiments on fourteen different assemblies of fillet weld joints produced by both underwater and in-air welding. Mechanical properties of underwater fillet welds in terms of strength, ductility, and failure modes are investigated in this study. Weld profiles, hardness distributions, and metallographic features of underwater welds are also examined. The study indicates that underwater fillet weld joints have larger strength but smaller ductility when compared with their counterpart in-air ones. The increase in strength ranges from 6.9% to 41.0%, and the decrease in ductility is about 50% for most of weld assembly types. Underwater fillet weld joints on corroded SY295 steel exhibit inferior ductility due to underbead cracks. Based on the test results, the paper proposes a practical method to model underwater welds in finite element analysis.