In case you are thinking of becoming an Underwater Welder, it is very obvious that you would need to get the welding certificate from a recognized institute. There are many welding schools which are accredited by the top boards which can provide you this certificate. As an alternate, you can complete Certified Welder Program provided by the AWS, that is, American Welding Society. You would need to give a test finally so as to get the certification though.
Pursue a Course at Underwater Welding School
You would need to learn the art of underwater welding and what is underwater welding before getting a certificate for the same. And, if you really want to get perfect in it, you would need to attend an underwater welding school. Keep in mind that you would need to have both the certificates; the diving one and the welding one, so as to get admission in this school and you would also practice both the things together here.