An Unfortunate Turn of Events is a treasure hunt in Velen. Aside from getting nice loot, you’ll also get to do some swimming and diving.
Name: Unsent Letter
Note that the rewards can be randomized, so you might not get the same loot as the one listed below.
Following contains quest spoilers. You’ve been warned
Find the corpse in the water.
Go north from the village of Lurch until you reach the bank of the river, then swim across to a small dock and harbor. Go to the dock’s right side, past the remains of a boat, and you’ll find a corpse lying in the water. Loot it to read the Unsent Letter.
Find the treasure chest.
Go east to the objective area, then dive down into the water. Drowners will attack you on the way there. Kill them and get to the chest on the river bed. Loot the chest to get your rewards and complete the quest.
The chest is at the bottom of the river.