To thoroughly investigate the underwater motion characteristics of rod jets, the effects of liner thickness, material, and charge length-to-diameter ratio on the underwater motion characteristics of rod jets were systematically explored by combining experimental methods with numerical simulations. The results show that after entering the water, the rod jet undergoes head upsetting and experiences mass erosion effects. The effective length of the jet initially increases and then decreases during its motion, while its average velocity decays exponentially. Further analysis indicates that increasing the liner thickness and charge length-to-diameter ratio can significantly enhance the jets resistance to erosion and its ability to maintain velocity. The optimal range for liner thickness is 0.036D_k to 0.055D_k. When the charge length-to-diameter ratio exceeds 1.25, the influence of charge structure on the underwater motion characteristics of the rod-shaped jet gradually diminishes. Additionally, material density has a significant impact on the velocity decay law of the rod jets during underwater penetration: the higher the density, the stronger the jets ability to maintain velocity; when material densities are similar, the velocity decay laws of the jets tend to be consistent. The study also demonstrates that liners made of copper, tantalum, and tungsten are all suitable for underwater shaped-charge warheads. This research provides important theoretical support and reference for the design optimization of underwater shaped-charge warheads.