Stupidity: Jake throws Derek overboard to distract the piranha, then swims under the boat and doesn't get attacked, so why couldn't he just go downstairs the normal way and have Kelly swim over to him? They could have escaped a lot more easily.
Continuity mistake: The cliffdivers' shorts change from blue to white.
Mr. Goodman: The piranha hunt in packs. The first bite draws blood, blood draws the pack.
Trivia: In the opening scene with Richard Dreyfuss fishing on the lake he is singing to himself "show me the way to go home". This is a nod to the movie Jaws because it is the song that he sang along with the rest of the crew as they waited for the tagged shark to be re-sighted. (00:01:00)