Dynamic behavior of the bubble was investigated by high-speed camera and using the dysprosium lamp method for welding speed ranging from 1.0 mm/s to 6.0 mm/s during conventional underwater wet welding and mechanical constraint-assisted underwater wet welding. Process stability analysis was conducted by welding electric signal to predict the evolution of bubble features. For conventional underwater wet welding, the variation in bubble geometry over time is visually observed and its track morphology changes from a vertically upward track to a deflected track with increasing welding speed. The stable and sustained bubble with its structural integrity benefits from mechanical constraint for all welding speeds. Mechanical constraint can compensate for the insufficient bubble protection and successfully prevent further bubble-induced perturbation on welding arc. In comparison to onshore welding and underwater wet welding with mechanical constraint, conventional underwater wet welding indicates the most variation of welding electric signal. The variation coefficient exhibits similar result with welding speed for the exertion of mechanical constraint, whereas the difference in variation coefficient exists for conventional underwater wet welding. The direct observation over weld morphology is also demonstrated to show the feasibility of bubble control using the developed process. Discernible difference in weld penetration appears, displaying higher value with mechanical constraint than without mechanical constraint.