Legend of the Emerald Lady — Carolyn Keene

ON THE EXOTIC ISLAND OF ST. ANN, NANCY SEARCHES FOR SECRETS BURIED IN THE PAST!
Swimming, sailing, and snorkeling are all that Nancy, Bess, and Ned expect when they visit old friends at their nineteenth-century plantation, Sugar Moon. But when a strange man appears, brandishing a cutlass, and Nancy discovers traces of intruders, her curiosity is aroused. Soon she comes across a faded love letter with clues to the hiding place of a long-dead pirate's precious gift -- a fabulous emerald necklace.
Many colorful island characters have a serious stake in finding the treasure, and Nancy barely escapes from their traps, on land and at sea. Then, just as Nancy locates the necklace, it's about to slip from her grasp forever!