This page of the guide to The Witcher 3: Wild Hunt includes a walkthrough of the side quest - Nilfgaardian Treasure. This is the only treasure hunt quest (minor side quest) related to the island of An Skellig . Our walkthrough explains, among others where is the bay associated with the quest, where to find the body of the survivor and what is the contents of the chest with Nilfgaardian treasure.
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.