To start this quest, sail to the northwestern edge of Undvik Island and find the body of a sailor lying in a crashed boat. Take the Traveler's notes from the corpse.
Geralt found a document which grabbed his attention…
Precious Haul
Related points of interest
Quest stages of Precious Haul
1. Read the document you found.
Read the notes.
Traveler's notes
I swear, not in my whole life have I ever seen such waves. Ten, twelve, fourteen feet! And the wind - why, it shook the mast and battered the sails so they were near about to fly off. Rocks everywhere, currents treacherous... Helmsman said we'd pull through, that he'd seen worse and come out all right. Me, I'm not so sure. If the storm don't calm soon, this tub's going to break in two.
2. Find the sunken cargo using your Witcher Senses.
Sail a little north, dive, kill the sirens (13) around and take the contents of the chest.
Though the water was exceptionally cold, Geralt dived down and fished the valuable cargo out of the wreck.