Where is the the boat with the skeleton on it in Lego Harry potter?

In LEGO Harry Potter: Years 1-4, the boat with the skeleton can be found during the "The Leaky Cauldron" level. After you exit the Leaky Cauldron, head towards the river where you will see the boat. Interact with the boat to uncover the skeleton, which is part of a collectible challenge in the game.