Witcher 3 Nilfgaardian Treasure Walkthrough
Start Location: At the site of a beached shipwreck on the southeast corner of An Skellig, the large north-eastern island of the Skellige Isles.
As you approach the ruins located here, you’ll see that it is guarded by a level 14 Siren. After dealing with the monster, climb atop the deck of the ship. Loot the dead soldier’s corpse here for a Cargo List.
Read the Cargo List in the Key Items section of the inventory. This will update our objectives. Make your way to the objective marker directly to the east of your current location.
If you look out into the water from the deck of the shipwreck here you should be able to see a mast sticking up out of the water nearby. Swim over to this and dive underwater to locate the chest on the deck of a sunken ship below.
Opening this will complete the activity.