The dog
This is a very strange movie because of the way it handles the dog. The dog seems to be an important character in the story, has a well-defined personality, seems very close to the title character, and has several heroic moments. Then it's killed and never referred to again. At all. Bizarre. I think that's some really crappy storytelling.
"All this machine does is swim and eat and make little sharks and that's all." -- Matt Hooper, JAWS