China has independently developed 10CrNi3MoV high-strength steel, which has become a crucial structural material for manufacturing submarine pressure hulls due to its high strength, excellent toughness, superior explosion - resistance, and outstanding corrosion resistance. The response surface method was used to study the effects of laser power, scanning speed, and wire feed speed on the dilution rate and deposition angle of the underwater single-pass deposition layer, resulting in optimized underwater single-pass deposition process parameters. Based on this, the influence of the water environment on the formation, microstructure, and microhardness of the single-pass deposition layer was investigated. Compared to air, the faster cooling rate in the underwater laser melting deposition process leads to a larger deposition angle and a smaller dilution rate in the underwater deposition layer. Additionally, the microstructure of the underwater deposition layer is finer, which results in a higher microhardness. This research provides technical support for the study and application of underwater laser melting deposition technology.