Go to the end of the movie, right after the chef asks Carter if there were only 3 sharks and he takes his feet out of the water. When the last shot is showing (or near the last one) where its a helicopter view of the whole setup they had there. Look in the water a little off center to the left, does that not look like a shark on a motorized track? Could this have possibly been used for special affects? I may be wrong, but look at it, it looks like a shark on a motorized track.