Deep Star Six
(1989)

After Diane (Cindy Pickett) seemingly electrocutes the creature to death (and dies herself in the process), sole survivors McBride (Greg Evigan) and Joyce (Nancy Everhard) (as well as their unborn child) get in an escape sub and get to the surface as the base blows up.

However, after they reach the surface and deploy the emergency life raft, the creature makes a surprise attack. While Joyce floats away on the raft, McBride ejects the sub’s fuel and then ignites it with a flare gun, causing the sub to explode and kill the creature.

McBride eventually emerges from the water alive and unharmed, and he and Joyce begin to row back to civilization.

Thanks Tornado Dragon!