Treasure Hunt: Sunken Treasure is a Side Quest in Velen.

Head west of Blackbough and northwest of Hangman's Alley. Once you have reached the beach you will find a body washed up onshore. Loot the body and you will find a letter. Read the letter and then follow the marker underwater.

Once underwater cautiously approach the marker. Before you get to the remains of a sunken ship you will see two drowners. Kill them with the crossbow it should take only one or two shots. Once they're dead swim to the sunken ship and loot the treasure.