Because welding is dangerous, and underwater welding is far more dangerous, i would recommend you take classes instead of looking for answers here or anywhere on the internet.
The navy, and all navy seals receive welding training, including underwater welding. That's the fastest cheapest way to get training but it is also VERY difficult to be a seal.
Boatyards and ship builders sometimes hire apprentices, so that's an option if you live near the ocean in a big city or are willing to move.
There are welding schools throughout the nation, and all professional welders have a union which I recommend you join. They can direct you to better schools and companies that hire.
Though I am no expert welder, I have enough experience welding and underwater welding to know, this is NOT a hobby you can just experiment with and learn safely. It takes four years to become a certified underwater welder, and that's IF you already know how to weld.
My training was a very short six-week program paid for by Uncle Sam, and that was only rudimentary training for emergencies at sea. During that process, I discovered why underwater welders get paid so much. It is difficult, scientific, physically demanding, tremendously dangerous work. You DO NOT want cheap advice from the internet...