Revealing mistake: When the truck leaps off the ferry, the underside is seen, exposing the still-flaming 'roll cannon' opening in the rear floorboard. This cannon is what caused the rear of the truck to rise, sending the truck flipping through the air. (01:52:30)
Continuity mistake: During Claire's post mortem, Doug takes a picture of her face. Her head was tilted towards the camera as were her eyes. When the picture comes out, it shows that she was faced away from the camera. (00:18:20)
Continuity mistake: When the red Ford Bronco is driven off the ferry (ignoring all the impossible factors required for it to successfully do so), it flips through the air and contacts the water going backwards, its right/rear tail light entering the water first. The closer next shot shows the truck sink somewhat as it continues to flip backward, onto its roof. The very next shot shows the interior of the truck as it enters the water right-side up and going forward as water rushes in. Denzel lurches forward and down, driving his face into the water that is about waist level. The next exterior shot is underwater as the truck now lands in the water, nose first and right-side up. It then begins the expected slow dive, nose-first. (01:52:30)
Answer: Not a definitive answer, just an "allowable" excuse for this... The time surveillance team is testing their equipment and rigs and it's merely a coincidence that they happened to be in this area at this time.