Dr. Bates says that fish with teeth developed around 320 years ago. She probably means 320 million years ago.
Janet continually mispronounces Cretaceous as "Cetaceous".
When looking at the buoy display, Stella says No. 4 isn't working, yet the red light for No. 3 is the one blinking.
At one point, Peter says the Devil Fish is eight feet away. A few seconds later, before it turns away, he says it's 20 feet away.
When the Devil Fish attacks Dr. Bates, she trips and throws her ax overboard. Later, she picks it up off the deck, far from the side of the boat where she tripped.
During the intro scene with the boat, the distant shot and underwater view show enough light to see the underside of the boat and land in the distance. Scenes on deck take place in pitch dark.
An oil slick fire would do little damage to an underwater monster.
When Peter throws the beer can over the side of the boat to test the underwater scanner, it sinks far faster than possible.