According to the BLS[0], there's less than 3000 commercial divers in the US, which includes underwater welding. Further on with underwater welding, it's dangerous, painful work that has long term risks (such as vision damage).

There's also just "standard" welding as well, but its the same story - working more than 40 hours, constantly being burned by molten metal, weak pay/benefits, and both short and long term health risks that are simply not worth the compensation.

[0] - https://www.bls.gov/oes/current/oes499092.htm