Goofs

Richard continually beats the shark swimming across the reef. A shark can swim from between 25-45 mph, whereas most Olympic swimmers can only swim about 5mph. He would stand no chance.

Richard tells the captain of the arrived boat that mother taught him that "twelve full moons are one year." Well, twelve full moons would be some 29*12 days, that would be 348 days, which falls short of 365 days of one year. But, that is the best way they were having to keep track of calendar. Only that the time they tell would be shorter than the actual duration passed.

When the Tradewind arrives at the island, Captain Hillard says, "Keep them in your sights. They may not be civilized.", to which Richard replies, "Civilized. C-I-V-I-L-I-Z-I-D."