On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Deviled Water side quest.
Reward for completing the mission: 1200 XP.
In the place marked in the above picture, you will meet a monk group who cannot perform the ritual. The man on the boat should be baptized, but he doesn't want to jump into the water because it's too cold. One of the monks will suggest helping the man and pushing him into the water or scaring him.
Jump on the boat and take control of it, and Eivor will throw the man into the water. One of the monks will now be able to perform the ritual, and after a short while, the quest will end.