Charles Earle Funk describes this expression in his book A Hog on Ice and Other Curious Expressions. He says it originated from observation by early sailors that a turtle turned on its back is helpless and makes for an easy meal. It is natural that they would describe a ship that has been capsized as having “turned turtle.” The sailors on a ship that has been turned upside down are undoubtedly just as helpless as that turtle they intended to fix into a meal.