While the pressure aboard such a deep sea habitat would be higher than 1 atmosphere as that would help it to resist the weight of the water (although not as high as the outside pressure), depressurizing aboard the habitat makes no sense as the submersible would be at the same pressure as the habitat. The submersible could either decompress on the way up or dock with a decompression chamber on the surface.
(The whole premise of the movie is that they built an underwater habitat that deep, something that cannot be done. As this is a fictional movie it's very likely that if they could build the habitat they could also weld at that depth.)
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 the creature attacks the 3 remaining crew members trying to access the decompression chamber, Norris (Cindy Pickett) battles the creature. The cable that is holding the creature up is clearly visible in two shots.
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.
Despite the fact that most of the members of the expedition are members of the United States Navy, no military protocols are ever observed, other than the dog tags. No one is ever in uniform, and even McBride's full closet has no uniforms in it.