when you return to the tower in mission 6 i believe to get this key you have to climb a ways up its near where the closest bone charm is. It is close to where a wall of light is and after that wall of light there will be a bridge. anywho a guard is holding it so you can knock him out kill him whatever then he will have the key on him and you can just take it
Dishonored: Waterlock Side Door Key
From Orcz
