Horrors movies in the ocean that uses sea creatures as the “slasher”
- Jaws I and II
- The Creature from the Black Lagoon (not exactly the ocean)
- The Deep (it is not a horror movie, it is a thriller that uses the ocean extensively. There is a sea creature that plays a role though) Even though it doesn’t fit your criteria it is so good!
- Humanoids from the Deep. (God this one is just so bad, but it fits your criteria)