The line "the sun's nearly up" is said while sharp sunlit shadows indicate the sun is directly overhead.
While the teens are walking along the water edge you can clearly see the Rocky Mountains in the background. Not across any water.
The teenagers are going water skiing but there are no water skis in the boat and they still have regular cloths on. No swim wear.
According to Regg's friend, Skip, the island is 30 miles away and unknown. But when they get to the island its as big as a state. How could anyone miss it.
In the police station (which looks like a regular house) the sheriff is sitting behind a small framed square opening in the wall. This makes it look like he's ready to take your hamburger order. Normally police officers would be behind a desk.
When Morrie and Dotty are worried about how long their friends have been gone and go to the police station to report them missing, Dotty looks at her wrist twice, as if checking her wristwatch, but she isn't wearing one.
When the sheriff and deputy are riding in the boat, the sound effect is the sound of a boat idling even though the sheriffs boat is cruising at normal speed.