Jormungandr, the Midgard Serpent, a sea serpent, was described as having 'poisonous breath'.

So I say give the sea serpents in Valheim a ranged poison breath attack (similar too, but stronger than, a Greydwarve Shaman).

Make them a bit more threatening and dangerous than they are now.

And let them be included in monster raids if the 'red circle' of the raid takes in some coastal waters (build your base on an island? I can still get attacked).

If a lowly drake has a frost breath attack then the might sea serpent should have its poison breath attack!