I mean, it's done dry, but it's a limitation of from hydrogen embrittlement wet welding, not the divers. Mostly done by robots now, but people had the capability going back to at least the 80's.
See for example:
Hyperbaric Welding at 300 MSW They report "An offshore demonstration of a full scale tie-in 300 m.s w. ..." and "Welding procedures were previously qualified at 300 m.s.w. with some manual welding test performed at 400 m.s.w."
And even before:
Hyperbaric Welding at 320 Msw, Development of Adequate Welding Procedures, from 1980 where they report "Norsk Hydro today have adequate procedures and knowledge to specify and supervise tie-ins at 320 MSW."
Supposedly you can still hire someone to 300msw with enough money, see for example "
DNV-GL certified 300msw."