Steel Sword (Mastercrafted): Shipwreck Of The Southern Peninsula

Journey to the western side of The Mire territory in southwest Velen, and watch yourself as you head through the village of Condyle; it has been overrun by Cannibals! You’re headed due south of Condyle, down the grassy (and increasingly swampy) river banks, and into the river to the south.

Take a boat if you wish, or swim out to the sunken ship you can see the mast of. Use a crossbow if the Drowners are bothering you. Take a deep breath (or a Killer Whale potion) and swim directly down to the stairs leading into the hold. Though there are other chests to loot, the one you want is directly under the hold stairs. Grab the diagram and surface without drowning.