Jeremy Frank’s review published on Letterboxd:
I suspect James Cameron's The Abyss was so expensive, and so traumatically difficult to make back in 1989, that Hollywood basically refused to back another film with a similar setting for decades.
Underwater has the budget for a cast with a few stars, good CGI and strong production values. Ultimately though, its crushed beneath the benthic pressure of too many layers of genre. It sort of wants to be an action film, sort of a thriller, and sort of a horror film.
Maybe it's the fact that the CGI is doing so much of the lifting, but for a film about such cramped, claustrophobic depths it feels remarkably weightless. Some of that is also due to how unsuited the cast here feels. Kristen Stewart is a hugely talented actor. But I'm sorry, her millennial defeatism and I'm-not-like-other-girls bleached pixie cut feels utterly meaningless in an environment where pure mechanical and scientific knowledge are the difference between life and death. This is the rare film that would actually benefit from more jargony techno-babel. If nothing else than to flesh out the characters and the setting a bit.
At no point do any of the people in Underwater seem capable of more than going for a light snorkel in the tropics, much less suiting up to walk the floor of the ocean amidst explosions, falling debris and (why not?) icky mer-monsters.
There's so much cool stuff you can do with a setting this fundamentally scary and traumatic. Underwater can't be bothered to chose a specific direction to go in. So instead of doing 1-2 things well, it opts to do 6-7 different things poorly. I imagine the eerie rumbling sounds you hear throughout the the film are simply James Cameron groaning at this thing.