They say the submersible works by equalizing the internal pressure so it matches the external pressure. At 36,000 feet, that pressure would be 16,000 psi, far too much for any human to survive.
Falk would have died instantly as soon as he exited the sub at the depth where the monster had hold of the sub. He couldn't have swam around and shined the light at the monster.
When the diving bell was being pulled up the comment was made "at this rate of descent he'll need a decompression chamber" when it should have been "ascent." Update: Captain Crane said "ascent" NOT "descent."