Abstract: Local dry underwater welding tests of 304 austenitic stainless steel were carried out using a self-designed double-layer drainage cover and an adjustable ring laser as the heat source, and the underwater welds were compared with onshore welds. The effects of welding speed and power ring–core ratio on weld formation, microstructure, and mechanical properties were investigated. The results indicate that the central laser plays a leading role in enhancing penetration depth, while the ring laser has a positive effect on stabilizing the keyhole and promoting the escape of bubbles. Meanwhile, it is found that the ring laser has unique advantages in improving the formation quality of high-speed welding. Compared with the microstructure of onshore welds with the same parameters, the ferrite content of underwater welds increases, and the spacing of columnar dendrites decreases. As the proportion of ring laser increases, the austenite content in the weld increases; the equiaxed crystal zone in the weld center becomes larger; the directionality of columnar crystal growth on both sides of the weld weakens. The tensile strength and elongation after fracture of underwater joints are lower than those of onshore joints; as the proportion of ring laser power increases, the tensile strength and elongation after fracture of both onshore and underwater joints increase.