Continuity mistake: During the underwater scene where the zombie is fighting with the shark, the zombie's arm is bitten off by the shark. However, we see him with the arm after he loses it, and without the arm before it is bitten off.
Continuity mistake: In the end scenes in the church, our heroes can be seen throwing bottles of kerosene at the zombies. The first one explodes into flames on the floor, but the following three bottles are all thrown into space where there is no fire. The fire has vanished.
Continuity mistake: Paola Menard is in the village cottage alone, when the zombies begin to threaten her safety and attack. She then pulls the large wooden cabinet from the wall and pushes it against the door. However she pushes the dresser up against the door twice between shots.
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.