Kristen Stewart actually kind of pulls off carrying this thing, which was a surprise to me. I know a lot of people have really enjoyed her post-Twilight career, but I haven't really latched on to anything she's done. It feels pretty short, but not as short as it actually is at 95 minutes. The creatures were cool, I'm pretty sure. It was kind of hard to see. But for all its ups and downs I did basically enjoy Underwater, a solid mix of creepy monster, creepy environment, interspersed with tension-relieving dialogue. The only real problem is that it's very derivative. Not a massive issue in and of itself, the trouble is that it means every five minutes you think to yourself "Oh! This is just like X Movie (usually Alien)... I kinda wish I was watching that instead".
Final rating:★★★ - I liked it. Would personally recommend you give it a go.