You would make more as a welder in the private sector than as a job grade in the military. However, the military would clothe and feed you, train you and would pay you to learn this trade. They would also make you eligible for the Montgomery GI Bill to help pay for additional schooling after the military. There are a LOT of benefits to the military in addition to learning the skill
It all depends on how you look at it, becoming an underwater welder will take some time but the pay will be good. Joining the military the pay won't be great starting out but they give you a place to live and they feed you, on top of your regular paycheck and not to mention the other benefits like college, health care,traveling the world and expierence you get while being in. If you want to enlist I would suggest doing it, at a minimum you will do 4 years and if by then you dont like it you at least got to travel the world collect a paycheck all while gaining expierence that will make you look good when it comes to applying for other jobs, as well as pursuing either college or certifications with out having to pay for it yourself.