Wet underwater welding is a key to improving offshore construction and maintenance cost performance. Automatic wet welding with local dry space provided by a water curtain provides high-quality mechanical properties on weld metal. The most important factor in maintaining high weld quality is precise nozzle positioning. Light cutting by slit laser beam shows potential for shape recognition in air. Waterproof containers for the laser emitter and video camera are used for underwater seam tracking of weld line in turbid water. Acrylic windows on the laser emitter and video camera shortened the total light path length in turbid water and improved image quality. Laser light was dimmed by scattering in turbid water but detected groove geometry, enabling good seam tracking. A trial to get a clear image used a fiberscope camera in the welding nozzle and detected root gap width precisely. Welding conditions such as molten pool geometry were also detectable.