Revealing mistake: When Nick has swam back to his water scooter, he climbs up and drives away and the shark comes up behind him. But look carefully behind Nick as he is climbing. The nose of the shark is visible during the whole scene, right under the surface. This proves that the shark was a dummy, waiting to come up.
Other mistake: Several times throughout the movie people are looking into the camera. Mostly at the grand opening of Water World, and at the surf competition.