Leviathan

I went in thinking this might be like Jaws, giant creature threatens people trapped in a vulnerable place. But it's much more like John Carpenter's The Thing (with a healthy dose of Alien thrown in.) The threat attack from within the body, either mutating it or emerging from the inside. You have the heartless corporation, which is willing to sacrifice everyone, and the doctor (played by Richard Crenna) who decides to keep the crew trapped with the creature.

(There's a scene where Crenna is explaining the reasons one might want to modify a human to have the characteristics of a marine animal. He uses the same cadence for the speech he did when he was telling Brian Dennehy about how Rambo would eat things that make a billy goat puke in First Blood.)

Although, unlike Alien, both the doctor and the company appear to be acting to try and prevent the spread of the threat, rather than because they want more people to become affected and produce more mutations. The most unbelievable part is that the company would willingly take the stock hit by doing a faked up press release that their undersea mining operation had a structural failure that killed everyone.

(Well, that or the part where Peter Weller, Ernie Hudson, and Amanda Pays are trying to make it to a safe part of the station before it implodes, and Pays is the one struggling to keep up. This despite the fact the movie emphasizes that she exercises more than anyone else, including regularly running through the interior. Because she's trying to qualify to become an astronaut. But of course she can't outrun Hudson or Weller.)

I like that they actually let Peter Weller punch the two-faced, backstabbing CEO at the end, given that the CEO was played by a woman. Don't usually expect to see that, where the male protagonist just hauls off and sucker-punches a lady, even if she is a duplicitous scumbag.