In the beginning, when Austin is doing the swimming stunts. You will notice that he is under water, but when he comes up out of the water, he's perfectly dry. This happens a couple times, you'll see...
this part of the film was played backwards to give this effect. The original taping of the scene he was going in down into the water as they filmed. to get the "dry effect" they just played the tape in reverse. It is supposed to be this way.
When they shot that last part of the scene, when Austin comes out of the water dry, they shoot it backwards. What I mean is Mike (Myers) stands on that pedestal above the water, which is then lowered into the water, so when played backwards, he emerges dry. :)
If you're talking about the finale of the opening credits, when the swimmers lift Austin out of the pool, that was run in reverse. Just watch the sparklers.