There is a diving joke that has been around in the UK for some time...
Q How do you make a small fortune diving.
A Start with a large one.
Seriously, if you want good money diving, but without the danger, then you will not find that, unless you have the capital to buy a decent dive operation. I think that the majority of instructors will agree that there is very little money in instructing.
Big bucks go to comercial divers that do the REALLY dangerous stuff like saturation diving etc.. There are a couple of ex comercial divers round here, try finding the posts of Deap Sea Dan.
I don't know exactly why undersea welding is so dangerous, but I would expect that it is all to do with the equipment necessary to weld underwater, combined with task loading at depth, and also the general environment in which jobs like this are done in - they are generally done in cold dark, poor visibility, deep water.
Jon T