The long drop treasure hunt is one of the more challenging ones for those, like myself, who tend to rush through the notes and not read them correctly. This one requires you to pay a bit of attention to what you are doing.
The video guide below will show you the solution to the long drop treasure hunt in Far Cry 6.
Where Is The Key After Closing The Water Dam?
Once you have closed the water dam using the controls in the office, you will need to go and find the key. Walk back toward the main dam structure and take the lower path, closer to the water. You will eventually get to a part where the metal path has fallen into the water. Look down to the right and you will see a dead body hanging from the metal clamp. Carefully jump down and you will be able to pick up the key to unlock the door to the treasure.