Factual error: When Brian fires a rifle at the shark from the boat, he fires the rifle multiple times very quickly. The rifle he is using is a bolt action rifle, which must have a bolt part moved before each fire. He does not touch the bolt.
Factual error: In the last scene, the radio announcer in New York City states that the governor has declared a state of national emergency. Only the President can declare a national emergency, not a governor of a state.
Chosen answer: After the original zombie extra fell ill, the shark handler was made up to play the underwater zombie. The shark was tame, or at least enough to not attack the handler. The makeup would have been latex/oil based, so it would not have been affected by the water. I remember from the commentary that the filming took 1 or 2 days.