There are very few scenes in the film that were actually shot underwater, as production went for the "dry for wet" look. With most of the scenes inside the Shack taking place on soundstages and a tank measuring 130ft x 270ft.
According to the sleeve notes on the DVD, a dry for wet lighting test by Alex Thomson used an old army tank for the wreck of the Leviathan,and some make shift diving suits were made using padded football suits and helmets from 2010 (1984).The final suits in the film were designed by Steve Burg and built by Stan Winston's crew.
Near the beginning of the movie, when Tony 'DeJesus' Rodero's oxygen is about to run out, the captain tries to do something with a computer on board. Between 3D animations and strange interferences on monitor, we see Luxo, Pixar's logo, with a ball, like in the first ever short film made by Pixar Luxo Jr. (1986).
One of the characters gets the length of her shift incorrect. The opening narration text states "Day 87 of 90 Day Shift". However, at about 11 minutes, Bow says, "It would be just my luck to finish two months' worth of work two miles under, and have the whole place fall apart with just three days left."
In the final escape sequence, the remaining crewmembers use inflated flotation devices to pull their pressure suits to the surface, but once they eject from the suits and surface, the flotation devices are nowhere to be seen.
You can see a crew member, probably the camera assistant, visible in the lower left hand corner of the mirror when Beck and the doctor are talking when the disease first makes its appearance.
The creature in the movie was a by-product of bad Russian genetic engineering in their attempts to make a human fully adapted to marine life. They slipped marine-based mutagens into the drinks of the crew of the Leviathan which cost them their lives and the ship escaped the Baltic Sea. The Soviet government decided to send a destroyer to sink the Leviathan to try to stop the creature from spreading to anyone else. The creature itself is made up of several marine life forms and the most visible of its makeup is octopus, eel, lamprey, and viperfish. While strong and able to adapt to the harsh pressures of the deep ocean, the creature itself needs blood for nutrition in order to survive. The creature doesn't eat or kill any of its victims, but assimilated their bodies into a single entity which causes the creature to grow larger. Victims from the mutation process retain only their minds; their bodies and intelligence will become part of the monster itself.