Corrected entry: Several times in the film it is mentioned that the rig is on the ocean floor some seventeen hundred feet below the surface. Yet Lindsy swims without any protection from the wrecked submersible to the rig, and Bud and others swim from one module to another. The absolute maximum depth the unprotected human body can stand is less than six hundred feet, and that is in the case of superfit divers who spend years training for their record dives (and some die trying). At seventeen hundred feet the water pressure would crush an unprotected human being like a schnitzel.
Correction: For a moon pool to work, the pressure inside of the underwater habitat the pressure inside would have to be equal to the water pressure outside. 1700' would be 518 meters underwater, with a air pressure of nearly 53 times that of the surface. FYI The deepest scuba dive to date was 332.35 m. With scuba divers, you start to be at risk of oxygen toxicity at 6-7 atmospheres and symptoms increasing the longer the exposure. So the grew of the underwater habitat would have all died from oxygen toxicity pretty quickly, let alone the effect of nitrogen at 53 atmospheres on the body.
You're right that compressing normal atmosphere like that would cause problems with oxygen and nitrogen, but they're not breathing highly compressed normal air. When you go that deep, you breathe a special mix that has a much lower percentage of oxygen and nitrogen. Typically, helium is added since it is inert. Look up heliox and triox.
If they were all breathing the oxygen/helium mix used by deep-sea divers, they would all be talking like Mickey Mouse, and they aren't. Obviously, the filmmakers wanted to avoid this comical side effect of breathing in helium, but it still reinforces this mistake.
Correction: As a deep sea SAT diver I have worked on many rigs as in the film. The air pressure in the rig is the same as the water pressure. That's why the sea doesnt pour in through the moon pool. You can swim from the rig without a problem, I have done it many times with just a wet suit to stop the cold killing me. You cant swim to or from the surface though as you would change pressure to fast and your lungs would pop.