Because welding is dangerous and underwater welding is far more dangerous, I suggest you go to class instead of looking for answers on the Internet here or anywhere.
The navy and all navy seals undergo welding training, including underwater welding. It's the cheapest way to get training, but it's also hard to be a seal.
Shipyards and shipbuilders sometimes hire apprentices, so it's a choice if you live in a big city or if you're willing to move.
There are welding schools all over the country, and all professional welders have a union. I suggest you join them. They can direct you to better schools and companies to hire.
Although I am not an expert welder, I have enough experience welding and underwater welding to know that this is not a hobby you can do experiment and safety study. It will take four years to become a certified underwater welder, if you already know how to weld.
My training was a very short six-week plan paid by uncle Sam, which was just a preliminary training in the maritime emergency. In the process, I discovered why underwater welders get so much pay. It's difficult, scientific, physical, very dangerous work. You don't need cheap Internet advice...
