Factual error: When Christian Slater and Minnie Driver are trying to get out of the water before it reaches the transformer, there is a real big problem with this scene. If there really was a flood and it was rising that high, the city should have shut off the electricity to the entire city, to prevent any electric hazards that may happen in that situation.
Factual error: Current flows in a circuit. The guy on the metal antenna tower is similar to a bird sitting on a power line. No current flows through the bird. Furthermore, current would flow from the transformer through the water to ground. There would be no current flow up the metal antenna tower.
Factual error: In the scene inside Karen's house, the water level rises way too fast. The house is not in an isolated pool of water, but connected to the entire city. So for the water level to rise several meters inside the house would mean that the water rose several meters in the entire city. And that could not happen that fast. If it was a wave from the lake, the water level would drop again.
Answer: Because the Sheriff wanted to kill everyone, leaving no witness. Remember the old saying, "The Enemy of my Enemy is my Friend."