Revealing mistake: At the beginning, when Gail is attacked by a moray eel - we don't see it, but that's the assumption - and she is banged against the sunken wreck of a ship, and then, when she is released, her mouthpiece is not in her mouth and she tries to swim to the top as fast as she can - all of this is done by a stunt double, understandably, we can see this by her hair, which is much thinner and straighter than Jacqueline Bissett's hair.
The Deep (1977)
1 revealing mistake - chronological order
Directed by: Peter Yates
Starring: Nick Nolte, Robert Shaw, Jacqueline Bisset, Dick Anthony Williams
Continuity mistake: In the opening scenes, when David is chiding Gail for grabbing him through the porthole of the wreck, his right hand is gloved, then ungloved, then regloved..all in the space of a few moments.
Henri Cloche: If his tongue moves again, cut it.
Trivia: Author Peter Benchley has a cameo as a U-boat crewman in the prologue (extended version).