This movie is only by a couple scenes I think back on and have been searching for.
We are out on the ocean, on an ocean liner, a science vessel, or other ship of some kind. There is an unusual looking alien specimen, or sea creature being held within a metal compartment, in a glass tank or something. The guy opens the metal door, turning away to some science equipment, etc....but hears the creature stir behind the glass. He turns around and eases close to the glass gazing into the tank at the entity. After a silent pause, the creature explodes through the glass, slamming him against the other wall, and plasters him up against the wall, under it. Kinda a transparent material, with lots of tendrils (I think).
At the end, I think the man is possessed by the creature, and with his body doused with fire...he steps over the edge of the ship, and leaving the ship burning behind...disappears into the ocean, beneath the waves in the night.