Above Novigrad you will find a small hut surrounded by a group of drowners (level 34) (M1,10). Go inside and on the left side you will find a corpse and a letter near him. Then destroy the barrels with Aard sign. This will allow you to open the hatch to the basement, where you will find a body with a rusty key and a letter that triggers the quest.
In the chest you will additionally find 100 florens, don't miss it. After reading about the quest from the note go on a hunt for the sunken ship.
The ship - or rather a boat - can be found nearby, on the east near a small island. The closed chest contains partially random low value items - runes, alchemy ingredients and some coins.