Go to the small bay in southwestern part of Urialla island (M17,2). Defeat two sirens on 13 experience level. On the deck of the ship you will find a body of a castaway. Take a book from him and read it. Approximate location of the treasure will be marked on the minimap. Swim towards the ship mast and then dive. The chest is in the middle of the sunken deck. Inside you will find few valiable items and Nilfgaardian special forces insignia.