On this page of the guide to The Witcher 3: Wild Hunt, you will find descriptions of treasure hunt quests on the island of Undvik in the Skellige archipelago. These are minor optional quests revolving around obtaining the unique Bloed Aedd sword and locating the sunken treasure in Tor Gvalch'ca, among other things.
In this quest you must find a treasure that is under water. Start by exploring the surroundings of the lake located in the central part of Undvik island (M21,4). Be careful, there are, among other creatures, wraiths nearby.
After taking care of the monsters, find a boat and search it. You will obtain a torn piece of paper. Read it from your inventory. Now you can jump to the water and dive in the direction pointed by the game. The treasure chest is at the bottom and inside you will find, among other things, a unique Bloed Aedd sword.
In this quest you must find a sunken treasure. You must start in the north-western part of Undvik island, near the location called Tor Gvalch'ca (M21,5). Examine the shore, you will see a shipwreck.
Enter its deck and search the corpse you will find there. You will obtain traveler's notes. Open the inventory and read the document. After that, go to the place marked by the game, North from your current position. After reaching the destination, dive. The treasure you seek is at the bottom, sirens and other creatures might make it harder for you to reach it.