The key for the chest is inside a small locked hut located at the north-eastern tip of the lake, This locked hut has a waterwheel on one side.
Since the front door is locked, you have to stand on one of the lines next to the wall hole and use Odin’s Vision to see where the lock is.
Use an arrow to shoot and break the lock. Now go inside the house and climb to the upper “floor” and you’ll find the key on a large crate.
Dive back into the lake and unlock the chest to collect tungsten ingot