Here's why it fits your description:
* Underwater Science Station: The film features a group of underwater oil rig workers who are tasked with assisting in a top-secret mission involving a sunken nuclear submarine.
* Water Monster: The "monster" in the film is actually a highly advanced alien life form that is misinterpreted as a creature by the characters. It's a mysterious and terrifying entity that attacks the station and its inhabitants.
Let me know if you have any other film mysteries to solve!