This page of the guide to The Witcher 3: Wild Hunt includes walkthroughs of treasure hunt quests in the Kaer Trolde area in Skellige. These are minor optional quests related to, among others, finding the unique swords - D'yaebl and Inis
Freya Be Praised!
You will find a corpse near the road (M13,5). Search them and you will receive a key and a letter with clues about the location of a treasure. The chest is under water in the nearby lake. There are some drowners at the shore, but you can bypass them if you don't feel like fighting.
Inside the chest you will find some crowns and florens, a "D'yaebl" sword and some common items.
Pearls of the Coast
On the island with the lighthouse (M13,12) you will find a small pirate camp (level 14) on one of the hills. After defeating them, search their bodies thoroughly. One of them will have a note about a treasure.
Search for the chest at the bottom of the sea where the marker shows. It's under the stern of the sunken ship. Inside you will find a lesser rune and a mediocre "Inis" sword.