You have to understand. That movie was trying too hard to be a bad movie. Movies you laugh at because they are so bad that make them good, should be a natural thing. Like The Room. Wiseau tried so F'n hard to make a good film. He was totally serious about it. And because he tried so hard and was so serious, we find it extremely funny.
Megashark was trying too hard. It was so over the top outrageous. Like the shark taking out that airplane. Like outrageously bad and funny would be the scene in Piranha (the original) when the fish jumps out of the lake and latches onto the guys nose. Or when you can see the wires and sticks attached to them.
Megashark was funnily bad, but you could tell that's exactly how they were trying to make the movie.