To receive the quest, go to the coast in the north-western part of the location (M4,2) and take a letter from the body of a drowner. The task will be automatically added to your journal. As the name suggests, the treasure is sunken. Luckily, you don't need to search for it too far. It can be found about 50 meters towards the sea from the place where you have found the letter. You will find the Bounty Hunter's Gambeson in the chest protected by two level 10 drowners.
On the north-western coast of Midcopse, you will find a body of a killed smuggler (M4,3). Before you reach it, you must kill two drowners. It shouldn't be hard as they are on level 4. Take a letter from the body and a treasure hunt quest will be automatically added to your journal. Read the note and the approximate location where you can find the treasure will be marked on the map. It's at the bottom of the sea, near a shipwreck South-West from where you currently are.
The treasure is in a chest protected by 5-6 drowners on level 10. The best solution is to swim first to the part of the shipwreck above the water, dive quickly, take the treasure and return to the ship. If you do it quickly, the drowners won't react. In the chest you will find good trousers among other things.
Queen Zuleyka's treasure
You can activate this quest by picking the letter lying on one of the beds on a beach on the western coast of the location (M4,4). There is a large pirate's camp around this place which consists of about 15 enemies on level 7. You don't have to kill them all to take the letter. After you read the note, the approximate location of the treasure will be marked on the map.
From the letter you will learn that the treasure is located in one of the damaged shipwrecks. It can be found about 150 meters North-West from where you currently are. If possible, use a boat. First swim to the island with the noonwraith and dive for the treasure from there. There are many drowners in the water, so you must act quickly. The chest is inside the sunken ship. You must find a hole in the hull and use it to get inside. The treasure consists of medium quality boots and few crafting materials. The diagram is most valuable, it allows you to create a very good armor (at least for the beginning phase of the game).