On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Crushed Dreams side quest.
Reward for completing the mission: 1200 XP.
In the place marked in the above picture, you will find the fisherman's corpse and a note in which the man thanks to his wife for everything she did for him. Next to the letter, you will find a ring that was supposed to be an anniversary gift. It seems that the fisherman was murdered by bandits and did not have time to return home in the Picheringa settlement.
Head to the Picheringa and find the woman named Vera marked in the picture above - you will find her at the fish stall. Tell the woman what happened to her husband and give her the ring (optional).