Conventional laser-induced selective metallization (LISM) of polymers in air often suffers from pronounced photothermal effects, causing the activation line width to exceed the laser spot size, which limitation severely hinders the fabrication of high-precision and high-resolution circuits. To address this issue, a water-assisted LISM strategy was proposed. Using acrylonitrile-butadiene-styrene (ABS)/CuO·Cr?O? composites as the research object, the optimal parameters for underwater ultraviolet (UV) laser activation were investigated by integrating this water-assisted strategy with LISM technology. The results demonstrate that the water layer effectively suppresses the thermal accumulation and thermal diffusion during the activation process of UV laser, thereby minimizing the heat-affected zone (HAZ). Comparative analysis of the activation and electroless copper plating results obtained in water and air reveals that the copper layer fabricated via water-assisted LISM is smoother, denser, and more precise. Notably, the narrowest line width is reduced from ~40 μm in air to ~28 μm underwater. Furthermore, mechanism investigations reveal that underwater laser activation induces the reduction of Cu2? to Cu?, forming active catalytic centers for subsequent electroless copper plating. This approach significantly improves the precision and resolution of metal patterns on polymer surfaces, providing a novel pathway for the fabrication of highly integrated circuits and miniaturized electronic devices.