Not Only Eagles Dare
In this quest you must find a valuable sunken treasure. You start it by reaching a small island in the Spikeroog bay (M15,2). Be careful as the territory is occupied by drowners and muckixers. Fighting this monsters might prove to be difficult due to the fact that there are many of them. After securing the isle, find a corpse near which you will find a Key with a ring attached and a Note. Open the inventory and read the note.
Now you must dive near the place where the examined body was found. In this case you must watch out for Sirens swimming under water. It is wise to regularly eliminate them with the crossbow. The treasure you seek is at the bottom, use witcher senses to find it. After looting the chest, start quickly ascending to reach the surface before your air runs out.