Before we learn this (as it is only suspicion for a little while), you are treated to some very violent depth charges (as well as some other interesting attacks). The tension escalates to a level that has not been matched by any "submarine" movie in recent memory.
Anyway, you probably think it is crap, because the advertising is crap. What they fail to mention is that it is a script originated by Darren Aronofsky (of Pi and Requiem For A Dream fame). It also doesn't mention David Twohy being the director of Pitch Black, which was also a really fun little genre movie.
Just think "burger not steak" and you'll be in the right mindset to enjoy your meal. It's a fucking great movie.