The Rift - Aka Endless Decent - (Sci Fi / Horror) - 1989

A prototype, experimental deep sea submarine Siren 1 has disappeared. A search and rescue team is sent out .The crew find an air-filled underwater cave, where the government is covertly conducting illegal and horrific genetic experiments. The rescue mission soon turns into a desperate fight for survival, as the crew battle hordes of mutant sea creatures,
They discover there is a traitor within the team, who plans to sacrifice the entire crew to keep the sinister experiments a secret.