Continuity mistake: In the scene where George talks to Billy about the key incident in the car, in the overhead shot the car is speeding very fast. In the next closeup, through the window we can see the car is now going much slower. (00:27:05)
Continuity mistake: When Kathy and George spot Chelsea up on the roof, Kathy races up the stairs, her sweater buttoned up. In the next shot as she enters Chelsea's room, the sweater is wide open and unbuttoned. (00:45:10)
Continuity mistake: Towards the end of the movie after George falls, and the rest of the family is climbing down, we see (from top to bottom of ladder) Kathy, Chelsea and then the eldest kid, with the youngest boy still reaching out on the roof. Yet, in the next shot, the eldest kid and Kathy are on the ground, and Chelsea is on top of the roof again, jumping into Kathy's arms.
Continuity mistake: After Kathy gets her hair caught in the boat's rotor, she runs back to Chelsea, through a downpour. But when George appears, he is dry, despite having to have had to run through the downpour as well. (01:08:40)
Continuity mistake: When George is awakened by the dog barking, George walks down the hall with his shoes tied. In the next shot, as he exits toward the opened door, his shoes are untied. The following shot shows George walking across the deck with his shoes tied once again. (00:54:55 - 00:55:45)
Continuity mistake: When George kills the dog in the boat house, he leaves the axe behind. Later when he goes after Kathy, it's back stuck in a block of wood by the house.
Answer: He sees the balloon, and instantly assumes his step-daughter is inside the boat house, and hence is in danger of slipping and drowning (or perhaps already in such a situation). He rushes straight out to get there as quickly as possible. Checking her room, or telling his wife would take up time, not to mention he is probably running on adrenaline.