I can't really conceive of a worse sin for a monster movie than being too stingy with the monster, although this movie comes close by existing solely to capitalize off of the release of James Cameron's The Abyss later that same year.
By means of charming miniature submarines and buildings, man has conquered the ocean floor and is exhibiting his usual recklessness. In an attempt to hurry up and build a missle platform already, a team demolishes an underground cavern and releases some big crustacean thing. The thing starts beating up their subs and their clubhouse but never comes close to the body count resulting from basic human stupidity.
An airlock door crushes somebody because a part breaks, someone else backs into a compressed air dart, and Miguel Ferrer sets off several nuclear missles without checking with anybody because he misunderstands the office procedure manual. They spend most of the movie talking about how they're gonna decompress themselves enough to escape, so there's a lot of dialogue about rerouting valves and fixing circuit panels. "Are you saying we can use E-2438?" "Yes, that's right, E-2438!" "Hmm...that might work." Meanwhile there's a giant monster swimming by the portholes going "Yo! Hey! Giant monster over here!"
In order to get the thirty-foot critter back in the plot, they have to keep screwing up so that hundreds of gallons of water can flood the room to chest-level so the monster can sneak in and pop out of the water and eat somebody. The final time this happens they don't even think of some valve or whatever they have to deal with, the character just hears a knock and says "that must be so and so" and opens the door and FOOSH! Pathetic.
It's too bad, because it wouldn't have taken too much effort to bump this up a couple of notches, at least so you'd wander past it on cable and maybe watch for a while. I suppose it's worth mentioning that there's a nasty death by surfacing without decompression (ouch!), but it ain't all that.