The film doesn't try to reinvent the wheel or work any miracles. It's successful in a very basic way in that it engages from beginning to end, with an occasional effective scare, constant sense of dread and high-pressure tension.
Stewart stars as Norah, an underwater engineer on a drilling rig in the middle of the Mariana Trench. We first see her brushing her teeth as the lights around her flicker, accompanied by some dull thuds. She glances around, she plays a little with a spider in the sink, she isn't too concerned.
Then, "Boom!," her section of the rig implodes, blasting water all around her as she and another crew member narrowly escape to a temporary safe place. And, by temporary, we mean safe for the next 30 seconds or so.
Norah makes her way through the quickly mounting disaster, meeting up with other crew members (including T.J. Miller along for comic relief in a movie made well before personal controversy rendered him a casting no-no). The vessel's captain (a strong Vincent Cassel) hatches a plan involving big diving suits and walking on the ocean floor towards a safer zone. The group reluctantly goes along.
It's not an earthquake or volcanic eruption that has caused their work home to be compromised. Sea creatures of no explained origin, some the size of the chest burster in Alien, some of more Godzilla-type proportions, are menacing the rig and occasionally dining on humans. Jaws style, the creatures are slowly but surely introduced, until a finale that is monster-ful.
Eubank stocks his cast with talent (which also includes Jessica Henwick of Game of Thrones and John Gallagher, Jr.), a cast that can say a lot in just the way they look, so long periods of exposition aren't needed for us to immediately care about them. The movie never lets up, and while it doesn't possess a remarkable screenplay or anything resembling emotional depth, the action is fierce and the characters still register.
In fact, had the screenplay provided one more solid, apocalyptic twist or shift, this film could've qualified as something very good rather than just merely good. There were moments where I thought Eubank might offer up some sort of Twilight Zone shocker but, alas, the movie is just about people running away from sea monsters. They run away in relative style, but that's all that really happens.
The monsters themselves are mostly seen through dark, murky water. Normally, obscuring special effects and not allowing full sight of a monster would infuriate me, but it works here, much in the same way it did with the cave dwellers in The Descent. Because we witness much of the action as if we were part of the escaping crew, we often see the monsters lit by flashlights or through suit visors. It works, for the most part. Nuts to the studio (Fox owned by Disney) who decided to make this PG-13. It needed to be an R.
Stewart provides the mayhem with a nice center of gravity, creating a character who is clearly scared shitless yet realistically under control. She shaved her head for the role, but Norah would seem to have that haircut because it makes more sense given the living conditions rather than it being an attempt to look tough.
So, it's January, and January usually sees the release of movies studios have zero faith in (along with awards contenders getting their wide releases after limited runs, like 1917 and Just Mercy). Underwater definitely fits the bill as a long-delayed, question mark of a movie.
Fortunately, for genre fans, it doesn't suck like a January dumping ground movie. It's passably good, and I can safely say it has no moments I would categorize as bad. It also has none I would call truly great. It's a good, old fashioned, please-monster-don't-eat-me movie, and I will gladly take one of those every now and then.