'The old fisherman in the marsh is preparing for a fateful fishing trip. His apprentice has gone to fetch supplies, but should have returned long ago.'The Old Man and the Marsh is a sidequest you can encounter in the marsh, by entering the old man Sandarr's house and speaking to him about his apprentice Mani. This quest is linked to the other marsh quest The Hunters in the Marsh, in which Sami is a member of the thrall group they have captured.
Return to Sandarr after you have captured Sami, and he will reward you and end the quest.