Sugarcane Island (Which Way #6) by Edward Packard - 1978 Rare Vintage Paperback

The first-ever "Choose Your Own Ending" style book, Sugarcane Island has been published as part of several series, including "Adventures of You," Which Way," and "Choose Your Own Adventure." 

You are the primary assistant to a renowned scientist studying turtles in the Galápagos Islands when a sudden tidal wave washes you overboard, sending you through powerful, monster waves until you wash up on a seemingly deserted island. Who - or what - might you encounter on your search for help? With dozens of possible endings, the adventure is in your hands.