In order to unlock this location, you need to complete "Restoration". The bridge which will be built thanks to you will lead to Driftwood.
Go to the biggest island of the archipelago and then enter the cave opposite to the small harbour . Inside the first key is hidden .
When you exit the cave, turn right  and climb the next island. Right before the small group of houses, jump off the cliff on the left. The next key awaits you there .