The serpents mostly appear at night; encounters during the day are relatively rare. Also, you want to be as far from the shore as you can, in the center of the ocean. Stormy weather also helps. The spawns happen every ten minutes or so, but it's largely up to random chance.

Do sea serpents Respawn in Valheim?

Each zone has a chance to spawn a Serpent when a player sails through it. The chance is 5% when it's night time or raining, or 10% chance if it's both. A zone only attempts spawning every 1000 seconds so backtracking won't trigger more spawn attempts.

Are there sea serpents in Valheim?

Serpents reside in the Ocean biomes of Valheim. Rafts are too slow in deep water, and break easily against Serpent attacks. It is recommended that you build a Karve or Longship to successfully hunt these sea monsters.