Other mistake: As the plane is beginning to crash, Chuck grabs the watch with his girlfriend's photo in it, and somehow it survives everything, but how? He had the watch in his hand, but in many shots we can see Chuck with both hands open as he needs them to hold onto something. He didn't put it in his pocket, he didn't have time.
Question: Ton Hanks spends 4 years on the island, subsisting on nothing more than rainwater, coconuts, and fish. It's not shown, but possibly he might have caught some birds, too. He lost weight on that diet, but wouldn't the lack of certain vitamins and nutrients given him scurvy, pellagra, beri-beri to the point of death? (Scurvy takes just a few weeks to set in.).