I'm relatively new to the game – iron age, 30hrs. I have been sailing with a carve all the time, explored many islands, spent so much time at seas- as far as the plains biome. I have built my 1st longship and set sail like nothing is happening, as i ran into some kind of island and on it were some shells. When i mined it the island sank down and a serpent appeared for the 1st time. I didnt know there were even enemies in the sea. I sailed away from it. On my second trip to the seas i was ambushed by 2 serpents one after another and they sunk my ship down. Atleast i killed one but still.
Conclusion: if i hadnt mined that shell i would have still enjoyed the seas without any worries at all, this has ruined my sailing experience forever! It used to be so chill spending all day on the ship exploring and now i have to fight those bimbos every damn minute. Did this happen to you too?
Source: https://www.reddit.com/r/valheim/comments/syu335/serpent_rant/