In this movie, professional divers, or scientists, dive to a very deep portion of the ocean floor, perhaps the Marianas trench. I can't recall the purpose. There is storm and the divers lose contact with the surface. A military or other vessel can't reach them until the storm subsides. Strange things begin to happen on board the submerged craft.
That should be enough. I don't want to spoil the amazing ending. I've been incorrectly thinking for years it's "The Deep" but just discovered that is not the one I'm thinking of.