Abstract: The surface formation and porosity of underwater laser welding seams largely determine the strength of the welds. To investigate the effects of various parameters on the melt width, melt depth, and porosity of the welds, and to explore the influence of the most significant parameters on the mechanical properties of the joints, an underwater welding platform was independently built to conduct underwater partial dry laser welding on 304 stainless steel. Through orthogonal experiments, it was found that the parameters with the greatest influence on the melt width and melt depth are welding speed and laser power, respectively, while the parameters with the greatest influence on porosity are welding speed and water depth. It was also found that boric acid concentration has no significant effect on the melt width, melt depth, and porosity of the welds. Based on these findings, the influence of laser power and welding speed on the mechanical properties of the joints was further explored. The results indicate that when the laser power is high, it is easy to create depressions that lead to stress concentration and deteriorate the joint performance. When the welding speed is low, the keyhole becomes unstable, easily leading to porosity and other defects that degrade the joint performance. Under the experimental conditions, a satisfactory welded joint was obtained with the optimal combination of process parameters being: welding speed of 0.9 m/min, laser power of 2.8–3.0 kW, shielding gas flow rate of 25 L/min, and water depth of 20 mm.