Continuity mistake: When Dan walks down to a podium and looks over to see the dolphin knocking at the gate, the tee-shirt he is wearing changes from being plain white with a tag badge on his left, to a white shirt with a large blue stripe across it, with a tag badge being on the right.
Continuity mistake: When two divers emerge from the water, one pulls their goggles up onto their forehead and the other pulls his goggles down to round his neck. Then the divers and Mike start talking about 'no overtime', and between shots the goggles around the divers neck vanish.
Revealing mistake: When the shark goes on a rampage at Sea World, whenever the fin goes above water, it is blatantly obvious that the fin is fake, and it is doubly obvious in some shots when we can see the welding line running right down the fin where the crew fused the two sides of the fin together.
Revealing mistake: About two-thirds of the way through the film, there is a wide shot of Jaws swimming. Keep in mind that a shark swims by the moving of its body and tail. It's quite obvious to see that Jaws is a matted-in puppet as his tail moves only once and he is propelled a long distance. Also note the fact that the tail move is quite mechanical and not the easy motion a real shark would make, and his body doesn't move at all. The shark in Shark Attack 3: Megalodon looked better than this.