On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Sunken Hope side quest.
Reward for completing the mission: 1200 XP.
In the place marked in the picture above, you will meet two girls: Runa and Rilda. When they see you, they will ask if you can swim. Their dad jumped into the water in search of the treasure and has yet to surface.
Dive in, collect the silver bracelet and return to the girls. You can give them back the bracelet or keep it for yourself. You can also tell them the truth about their father who drowned or lie that you didn't find him.