The 39 Clues Book Nine: Storm Warning by Linda Sue Park

From chapters.ca:
Throughout the hunt for the 39 Clues, Amy and Dan have encountered some of the darkest aspects of history... and had to deal with the role their family played. But are they ready for the truth? In this thrilling ninth installment, Amy and Dan hit the high seas as they follow the trail of some infamous ancestors to track down a long lost treasure. However, the real prize isn't hidden in a chest. It's the discovery of the Madrigals' most dangerous secret and, even more shockingly, the true identity of the mysterious man in black.