Other mistake: The dead Killer Whale washed up by the lighthouse was visible from the teens AND in their line of sight while they sat on the shipwrecked boat, also their Catamarans they pulled onto the sand were literally 100 feet away - as seen in the wide shot, but yet it was a surprise when Tina and Eddie come upon it. There's no wrack line showing in the sand so the whale didn't "just wash up "
Other mistake: When the shark grabs and eats Marge, there is no blood at all.