In spite of the opinion that Underwater is just a cheap Alien knockoff, there are several redeeming qualities to this deep sea creature feature, even if you might have to squint through the grime to catch a glimpse of them.
There is a level of authenticity to the movements of the actors due to the fact that the suits they're moving around in weigh a staggering amount, roughly a hundred and forty pounds. You can almost feel this in their motions and it bleeds through into acting that would seem otherwise subpar compared to most movies of this visual caliber.
As far as the actual dialogue and acting I found myself wondering whether there was even a script. It really seemed as if the actors were given some amount of free rein to adapt their dialogue and delivery as they saw fit, sometimes to almost laughable effect. Also, none of these characters will really stick with you to any capacity as they obligatorily get booted out of the script from time to time.
You have such gems as Goofy Guy, Strong Silent, Innocent Asian, Resilient Cynic, and lest we forget... Bearded Leader. We don't talk about Skittish British. But really, the star of the show is the threat, and by the end it feels fully realized, even if the characters don't. But then again, this is the template for the modern horror movie of this design. You can't fault it too much for following in the footsteps of its predecessors.
The point is that none of this sticks but it's moderately fun, totally worth renting for a couple of bucks, and the CGI effects really make it feel authentic. Bonus features include watching Kristen Stewart run around half naked and watching T.J. Miller care for a stuffed rabbit like it's a living thing because he's really, really into Alice in Wonderland.
It's flawed but I personally love it.