I started watching this film last night. I switched it off after 50 minutes of poor story telling and tedium.
It seems the director mistakes character development with brushing teeth and saving a spider without any context.
Nice little beasty - I got that far. However, because of the terrible pacing and complete lack of character investment and suspense or even environmental geography, it was all rather tedious and I figured I had better things to do... like going to bed.
To see what I mean, take two films this film was clearly trying to copy in certain ways (fumbling badly the corridor scene in "Alien" by misunderstanding its purpose) - "Alien" and "The Abyss." They are both beautifully made films. However "Underwater" drowns in the director's immature (in story-telling terms) race to get to action.
For watery creature features, please watch, "Jaws," "The Abyss," or "Sea Fever" rather than this. Those three are good, well-told films.
Some will like this film - I like some poor films myself. I'm glad for those who like this regardless of it being a prize stinker. I wish I could have found some enjoyment in it. The only thing positive I can say is that, the beastie looked nicely designed.