Michael, I am not in underwater welding, but, my son had looked into this years ago. At that time, his interest was working on ships. I believe there would also be positions in the oil industry. As you know, this job has two components: Underwater. and. Welding.
In order to perform any job underwater, you need to be able to scuba dive. Beyond that, you will need to be able to stay calm and think your way through when problems happen. You cannot panic underwater! My son was a diver, and really enjoyed it.
There are many different types of welds, and types of material that welders work with. Working in job placement, I have met many welders. They were very knowledgeable and took great pride in their work. A good weld is almost like art - it is a skill. I encourage you to learn all you can about working with the different types of metals.
I also encourage you to look into a career with the US Navy. There are lots of opportunities there, and, better yet, they will train you, at no cost to you. You will have marketable skills when you leave the Navy!
Other underwater careers would be in Environmental research, teaching others to dive, salvage operations (bringing up ship and even aircraft wreckage) and things like that.
I think this is a cool career, and wish you the best as you continue to explore it!