Continuity mistake: When they are chasing the suspect, Tom rams into the suspect's car and the driver's side hubcap flies off. Tom's dad says "pull up to him." He aims at the car with his gun and the hubcap is back on.
Revealing mistake: The police car that gets turned over in the tunnel explodes just before impact by the pickup truck.
Continuity mistake: The damage to the front of the bad guys' car in the opening chase changes considerably between shots throughout the whole chase.
Continuity mistake: When the fleeing suspect's vehicle broadsides a pickup truck, the front bumper falls off (chrome and two rubber bumpers) exposing the bumper support underneath, and the two driver's side headlights are completely gone. After the car goes over a bridge, the front of the suspect's vehicle is shown again and the bumper is back on the car, the chrome and two rubber bumpers are intact, along with the headlights. Then as the car takes a left, the right front side of the vehicle has the hood popped up, and different damage to the driver's side.
Continuity mistake: Near the beginning, at the end of the car chase the murderer's car both cars flip multiple times and the trunk flies open. However, when Bruce Willis sees his father on the gurney dead, cops open up the trunk and a body falls out.
Answer: Because they'd be completely useless if he allowed them to get soaked by the water.
Tailkinker ★
But his head comes out of the water. So the shells got wet anyway.