Abstract: Research on the underwater motion of shaped charge jets (SCJs) remains limited, leaving the evolution patterns and influencing factors of jet velocity and morphology in this medium insufficiently understood. Furthermore, established kinematic equations describing jet motion in water remain scarce. To address these gaps, a combined numerical and experimental approach was employed to investigate the evolution behavior of jets formed from five distinct metallic liner materials—aluminum, titanium, zirconium, iron, and copper—in both air and water. A three-dimensional Arbitrary Lagrangian-Eulerian (ALE) model of jet flight was established based on ANSYS finite element software to investigate the velocity and stress variations of jets in air and water. Concurrently, an experimental platform for jet flight was constructed, and the dynamic evolution processes in both media were recorded using high-speed imaging. The results demonstrate that jet velocity undergoes significant and rapid attenuation in water, decreasing by approximately 80% within 4 times the standoff distance, and the attenuation rate is negatively correlated with material density. Moreover, based on experimental and simulation results, combined with hydromechanics theory, a mathematical model was developed to predict jet velocity attenuation in water, achieving a Mean Absolute Relative Error (MARE) of 9.8%. Based on this model, the primary material factors influencing the jet velocity attenuation process are identified, with the weighted influence ranked as melting point, strength, and density. Additionally, the density, strength, and plasticity of the liner are the key factors governing jet evolution behavior in water. Specifically, high density and high plasticity effectively enhance the jet's fracture resistance and significantly delay its dispersion process. Notably, the resistance force acting on the jet tip in water can reach up to 2000 MPa, which intensifies jet fracture, leading to an increase of over 41.4% in the degree of jet dispersion compared to that in air. Furthermore, water significantly inhibits the oxidation energy release of the reactive materials via oxygen isolation and cooling effects, rendering their energy release characteristics similar to those of inert materials. These findings elucidate the influence of material properties on jet deceleration, fracture, and dispersion in water, providing a theoretical basis for the optimized design of underwater shaped charge warheads.