What is the deepest a man can weld under water?

In a pressurized diving suit the limitations will be the maximum depth the suit can withstand; since the deepest dives are accomplished in full "space suit" like outfits.

The overall depth of welding may be unlimited through the use of robotic arms on Deep Submersible Vehicles equipped with welding gear. There are different types of welding in which special gas displaces the ordinary atmosphere in the immediate area the weld is taking place. So, in order to accomplish a weld a extreme depths the Welder would need to transport not only themselves, but the necessary welding rods and special gas cylinders. All these factors begin making fabrication at the surface more economical and practical, with the final structures submerged to "the bottom" and assembled in place.