The Devil's Mouth

The basic premise has considerable potential because it combines two fears that already work perfectly well on their own: being trapped underground and knowing that something dangerous is moving through the water. Jeff Wadlow uses that combination to build a straightforward survival film, with few complications and a first half that maintains some tension through confined spaces, darkness and the feeling that any mistake could leave the characters without a way out.
Kathryn Newton and Lana Condor are the most recognizable faces in the group and also the ones who hold the film together most effectively. The relationship between their characters tries to add something beyond a simple succession of attacks, and at times it succeeds in making us care about who is frightened rather than merely when the next threat will appear. The rest of the ensemble, however, is considerably less defined and mostly functions as support within a very familiar structure.
The threat itself also loses some of its power once you start thinking too much about it. Accepting the rules of a shark movie is one thing, but it is difficult not to wonder how an animal like this could survive inside such a small and apparently isolated cave system. Some species can tolerate freshwater, but the film asks us to accept too much without much explanation, and that logical weakness slightly undermines an environment that should feel frightening precisely because of its apparent realism.
It does not help that many situations are easy to predict. The film relies on very familiar survival and shark-movie devices, and several sequences practically reveal their own mechanisms before they play out. Once a story like this makes its machinery too visible, suspense weakens because we stop wondering what will happen and simply wait for what we have already guessed.
Still, I would not call it a disaster. Some attacks are handled effectively, the underwater darkness creates genuine claustrophobia, and the pacing is solid enough to keep the film from becoming particularly dull. Even when it fully embraces B-movie logic, that exaggeration can work as pure entertainment. The problem is that it often feels like a variation on ideas we have already seen done better.
That more or less sums up my reaction. The Devil's Mouth combines caves, water, claustrophobia and shark thrills competently, but never turns those ingredients into anything truly distinctive. It works while you are watching it and offers a few tense moments, yet its predictability, lack of originality and some questionable logic ultimately weigh it down. Entertaining enough, but far too formulaic to be memorable.