Factual error: After George and Kathy leave the Doctor's office they drive through town. When they are talking in the car there is a shot where, next to George's face, you can see the sign of the town hall where the scene was filmed, 'Village of Antioch' in Antioch, Illinois. (00:46:00)
Factual error: There is a poster for Whitesnake on Billy's wall. This movie takes place in 1975. Whitesnake didn't form until three years later.
Factual error: There is an Alice Cooper poster on the wall of the boy's bedroom. The logo on the poster was not used before the 2000's.
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.