Character mistake: When they're reviving Lindsey, Bud screams "fight" and slaps her. watch as she closes her eyes on the second slap while she is supposed to be dead.
korporal koolCharacter mistake: When Bud is doing mouth-to-mouth on Lindsay, he's not pinching her nose closed, so all the air he's breathing in is going straight out her nose.
kh1616
Answer: The crew are working at that pressure and only need diving suits and submarines for long periods in the ocean and to breathe longer. Hence when they return to the surface in normal conditions, a slow return to the surface is required to avoid their bodies developing a condition called the bends. This did not happen at the end as they were protected by the aliens and their ship.