On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for The Comb of Champions side mission.
Award for completion of the mission: 1 200 XP.
In the place marked on the picture above you will meet a woman named Bil. She is accompanied by one man and it seems that they are both looking for something.
When you talk to Bill, it turns out that the woman has lost her comb and is waiting for someone to help her find it. In return, she offers his company and gratitude.
You will find the comb in the water, but you will have to dive very deeply. After jumping in the water, turn on Odin's Vision to mark the position of the object you are looking for. Remember that if Eivor stays underwater for too long, she will start to lose health points.
After finding the comb, talk to Bill again and follow her a little further to her makeshift camp. Here you will be able to talk to her again and if you want, spend some free time with her.