Leviathan
(1989)

While hiding out from the mutant creature, Elizabeth (Amanda Pays) decides to bring up a Tri-Oceanic Corp financial report, and in it, she, Steven (Peter Weller) & Jones (Ernie Hudson) find out that their boss Martin (Meg Foster) had just recently told her fellow superiors in the company that the facility was destroyed in an accident, and everyone died. She did this because she wanted to keep the mutant trapped in the facility, due to the lives already lost to it, and she shamelessly felt the three of them would die as well. 

The mutant then damages some vital systems, dooming the facility to an imminent implosion, but the trio escape both the facility and the mutant by going up to the surface in their diving suits. The facility implodes, presumably taking the mutant with it.

When they surface, they use a flare to get the attention of a Coast Guard chopper that was searching for them, since they received their SOS signal that they sent earlier. As the chopper comes down to pick them up, the mutant appears. Jones sacrifices himself to the mutant so Steven and Elizabeth can get on the chopper, but before Steven gets on, he chucks a demolition charge down the mutant’s throat, blowing it to smithereens.

Steven and Elizabeth are brought to a Tri-Oceanic Corp oil rig, where they meet Martin. She tells the two that she knew that they would make it, all the while smiling insincerely. When she asks how they feel, Steven casually punches her in the face as he walks past her, knocking her out, and says “Better. A lot better.”

Thanks Tornado Dragon!