On this page of the Assassin's Creed Valhallaguide, you will find a detailed walkthrough for Last Flight of the Gyldan Sparrow sidequest.
Rewards: 1,200 XP.
At the location shown in the image above, you will meet people who are lamenting over the loss of their cargo, which went down with one of their ships.
Jump into the water and swim to the spot marked in the image above. Dive in, collect the items and return to shore. Talk to the people waiting on the pier to proceed to the second part of the task.
Stand on the pier and catch a fish. In its stomach, you'll find a necklace that has considerable value in silver. Talk to the woman and give her back the necklace to complete this task.