I'm unable to pick up the Gauldur Amulet fragment in Geirmund's Hall [duplicate]

I'm unable to retrieve the amulet fragment of the Gauldur Amulet in Geirmund's Hall. This most likely is the result of me clearing the dungeon without the quest a couple of hours ago and perhaps not picking up the quest item back then. Anyway, the waypoint leads me to a place in the shallow water of the bossfight chamber and indicates something to pick up but it isn't there.

Can anyone please help me and tell me how I can solve this problem?

This questline is currently bugged. If you cleared the Gerimund's Hall dungeon before receiving the quest, the boss' body will vanish after you defeat him, thus preventing you from retrieving the amulet fragment. Unfortunately, until Bethesda patches this bug, there is no way to complete it.