I've had two qwests that direct me to the same place looking for 2 diferant people and though the compass says thats where they are...I see nada!
Its the corner of Robbers Cove that has a chest in it and looking up you can see sky. I've tred swimming underneath the area,used the "placeatme" and "moveto" commands and nothing.
Its the Companions "Rescue Mission" and the Forsworn Conspiracy (I think) Both keep me from advancing further
though. The skeletal hand swimming like a salmon in the cove is cool tho. Needs a good stick poking imo.

Try waiting outside the cell for 31 days then go back. Should reset the dungeon.

Also if one of your quests is a radient quest you can go back to the quest giver and quit it.

Thanks I'll try both and see what happens.