Richardson performs electric arc welding outside of the station, at the bottom of the ocean, several thousand meters deep. At present, underwater welding is not technically possible deeper than approx. 1000 meters (3300 feet) underwater.
When Snyder is accessing the computer system for the procedure to secure the missiles, the computer asks him to define "abandoned." The first choice offered reads, "Abandoned for repair or or removal," with the word "or" duplicated.
When Captain Laidlaw is caught by a malfunctioning hatch and orders his crew to abandon him, he intentionally floods the chamber in order to force them out. As the water level raises, you can see that surface is not horizontal, indicating that the camera was tilted. Also, the underwater shots show the surface calmer and much closer to the captain's face, so the actor would not be held underwater.