Treasure Hunt: An Unfortunate Turn of Events is a Side Quest in Velen.

Travel to the abandoned site, near the bridge, directly south of the Devil's Pit.

Jump down in the water and there you will find the wreckage and a body washed up on shore. Loot the body and read the unsent letter.

After you read the letter, swim underwater and you will encounter two drowers. Shoot them with your crossbar, it will take about two shots from your crossbow to kill them. Once they're dead swim to the chest and loot it. After you've looted the chest you will finish the quest.