The first thing that really hurt suspension of disbelief were the scenes with the swimmers effortlessly swimming down. After all, a split second before they didn't even have masks on!
I had the same thought when I saw the scene, but then I noticed a continuous stream of bubbles from their feet. It gave the impression of "thrust boots" of some sort, propelling them down. They'd certainly need the help at that depth!
Maybe they just whipped the masks into place when they hit the water, James-Bond-Thunderball style?