 FACT: When the crew enters the sunken submarine, they have to open  numerous locked hatches before they come into an area with the dead  sub crewmen. In one shot a crab is seen scuttling out from inside a  dead crewmans mouth. Considering that the submarine had been  submerged for a relatively short time (less than 24 hours), it is  highly unlikely that a crab would have had the time or ability to  make it inside the sub to begin with, let alone to the core of the  submarine to reside in a dead crewmans mouth.